Traitor Legion Loyalists

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COMPLETELY 100% LOYAL.

"Thou shalt not follow a multitude to do evil."

– Exodus 23:2

"The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son."

– Ezekiel 18:20

Traitor Legion Loyalists are the rare few Space Marines who, despite hailing from the Traitor Legions, stayed loyal to the Emperor and the Imperium, proving that despite what some characters in 40K think, having geneseed from a suspect source is no sure indicator of bad character. They are a very rare group of individuals given the initial purges of Loyalists elements from the Traitor Legions at the Battle of Isstvan III or by internal purges such as those enacted by “The Brotherhood” in the Word Bearers. While most of the information below has just been hints for now, this is a matter that has been popping up every now and then for the last few years.

As it happens, many of these Traitor Legion Loyalists during the Horus Heresy who did not end up in the annals of history became Blackshields, the precursor to the 40K equivalent of the same name where a Marine with no emblems or markings can petition to join with the Deathwatch on a permanent basis, no questions asked. The term "Blackshield" by itself leaves it open as to whether the individual Marine seeks to redeem himself OR his chapter if it went rogue or was destroyed. Although Blackshields are a rarity in 40K, there were enough in 30K for such a tradition to begin and survive to the present day in the Imperium. Many others were recruited into the Knights-Errant, a special force whose members were handpicked by Malcador the Sigillite and would later develop into the Grey Knights and the Inquisition.

Those groups with enough numbers, or perhaps ties to the right people, were able to pass themselves off as successors of other Chapters after the Second Founding. Or their founders purged and changed their own records, so that over time, no one within the Chapter knows who their true progenitor is (which can lead to some fun times when an Inquisitor comes across evidence that it occurred).

Generic armies of them can be fielded in 30K with the Orphans of Betrayal Rite of War. As is fitting for a rogue, largely unsupported force, the rite doesn't provide many bonuses unless you're fighting Traitor versions of the same Legion you're fielding.

Known Loyalist Members of the Traitor Legions

Note: Characters whose names are listed in italics have received rules in some form in FW's Horus Heresy releases for the 30k era. Additionally, Marines who died or went missing in action (without returning later during or after Horus Heresy) in the Great Crusade, like Hastur Sejanus (whose death in fact would be the immediate catalyst for the treason of Horus), don't count as while they are all technically loyalists there was no meaningful distinction at the time.

• Death Guard
  • Nathaniel Garro (became the first of the Knights-Errant, first to bring word to the Imperium about the Horus Heresy)
  • Andus Hakur
  • Meric Voyen (former head Apothecary of 7th Great Company. Defected with Nathaniel Garro. Renounced his vows as an Astartes following 'The Flight of the Eisenstein', slain after being possessed by the Lord of the Flies)
  • Huron-Fal (Venerable Dreadnought, chose to self-destruct rather than be killed on Isstvan III)
  • Tollun Sendek (Slain by the Lord of the Flies on Luna)
  • Ullis Temeter (Killed on Isstvan III by Huron-Fal's overloading reactor)
  • Helig Gallor (Recruited into the Knights-Errant)
  • Bajun Kyda (Recruited into the Knights-Errant. Killed at Proxima Majoris)
  • Crysos Morturg (Technically died during the Battle of Isstvan III, but was rescued and rebuilt by Calleb Decima. He became a Blackshield thereafter, ended up surviving the entire Heresy, and was made Captain of the 108th Independent Company. He and his Company were eventually sent out to monitor and patrol the Ghoul Stars. Notably, this was over fifty years after the Heresy ended, implying at least some surviving Traitor Legion loyalists were utilized by the Imperium to form "Independent Companies" instead of Chapters. Strangely enough the planet they were stationed on became the homeworld of the Death Spectres. A Raven Guard successor. Though there is at least one other chapter, who favors stealth combined with massed firepower. The 108th then possibly evolved into the Marines Malevolent.)
  • Erud Vahn (Became a Blackshield during the Horus Heresy)
  • Juball (Died on Isstvan III)
  • Solun Decius (Succumbed to Nurgle's Rot, becoming the daemonic entity calling itself "The Lord of the Flies")
• Emperor's Children
  • Saul Tarvitz (Called "The First Loyalist" for being the first to recognize the traitor legions' betrayal and helping to send warning to the Imperium. Currently MIA, though the authors have stated he died during the last bombardment of Istvaan III)
  • Callion Zaven (Recruited into the Knights-Errant. Killed on the Vengeful Spirit)
  • Argentus Kiron (Member of the Outcast Dead, slain on Terra)
  • Ancient Rylanor (Contemptor Dreadnought, KIA post-Heresy, nearly succeeded in killing Daemon Prince Fulgrim)
  • Rakishio (KIA in Sol System by traitor White Scars who failed to infiltrate Terra)
  • Solomon Demeter (Captain of 2nd Company, KIA on Isstvan III)
  • Lord Commander Vespasian (murdered by Fulgrim)
  • Lord Commander Abdemon (Member of the original 200 Emperor's Children Legion survivors -they had some form of non-Chaos related disaster- prior to reuniting with Fulgrim. Presumed slain on Isstvan III by traitor Astartes)
  • Callion Zaven (a member of the Knights-Errant).
• Iron Warriors
  • Warsmith Barabas Dantioch (14th Grand Company. Defied Horus' and Perturabo's orders to betray the Emperor. Led his garrison in the defense of his stronghold, the Schadenhold, during the the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus. Activated the Pharos navigation/communication beacon under Guilliman's orders, unintentionally drawing the attention of the Tyranids to the Milky Way Galaxy. Sacrificed himself in a glorious battle to save the forces of Imperium Secundus. Posthumously declared a HERO OF THE IMPERIUM.)
  • Iron Palatine Zygmund Tarrasch (14th Grand Company. Survived the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus.)
  • Chaplain Zhnev (14th Grand Company. Survived the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus.)
  • Sergeant Ingoldt (14th Grand Company. Survived the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus.)
  • Legionary Baubrista (14th Grand Company. Survived the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus.)
  • Legionary Toledo (14th Grand Company. Survived the Great Siege of Lesser Damantyne. Recruited by the Ultramarines to FORTIFY Imperium Secundus.)
  • Venerable Vastopol (14th Grand Company. Venerable Dreadnought. Died at the end of the Great Siege of Lesser Damantyne)
  • Ancient Khragan (14th Grand Company. Contemptor Dreadnought. Thought lost with dozens of others in a battle against the Hrud during the Great Crusade. Reappeared along with those dozens of others in a Loyalist Shattered Legion formation during the Constantinum Incursions of the Horus Heresy. Personally killed the traitors' leader.)
  • Warsmith Kyr Vhalen (77th Grand Battalion. Led 2,800 loyalist Iron Warriors during the First Battle of Paramar. Survived. Never stopped cussing the traitor.)
  • Veteran Sergeant Titus Avvon (77th Grand Battalion. Warden Officer of Castellum Stronghold Epsilon V. Survived.)
  • Legionary Zhinnon (77th Grand Battalion, 5th Counter-Armour Wing, 30th Squad. Fought for the Loyalists in the First Battle of Paramar. Survived)
  • Consul-Praevian Nârik Dreygur (114th Grand Battalion. Initially allied with the Traitors, he eventually defected to Cassian Dracos of the Salamanders. Led his Iron Warriors in a successful retaking of the planet of Mezoa from traitorous Alpha Legion forces.)
  • Ancient Orgol (114th Grand Battalion. Deredeo Dreadnought. Answered Nârik Dreygur's call to arms to attack traitorous Alpha Legion.)
  • Warsmith Auric Saxton (Remained fiercely loyal to the Emperor and killed the Lord Commander of the traitorous Emperor's Children during the Battle of the Harrow Ravening).
  • Warsmith Annovuldi (Remained fiercely loyal to the Emperor and allied with the Raven Guard and helped take the planet Carandiru from the Traitor forces. Was later sent to Terra by Corvus Corax.)
  • Tubal Cayne (A member of the Crusader Host. Recruited into the Knights-Errant. Killed while infiltrating the Vengeful Spirit during the Battle of Molech.)
• Luna Wolves
  • Garviel Loken (Survived the battle of Isstvan III, recruited into the Knights-Errant, rules are for Luna Wolf version only.)
  • Tarik Torgaddon (Killed on Isstvan III by Horus Aximand. Corpse reanimated to create the daemon Tormageddon, later slain by Garviel Loken during the Siege of Terra.)
  • Iacton Qruze (Recruited into the Knights-Errant, later killed by Horus at Molech.)
  • Severian (Recruited into the Knights-Errant. Eventually became Iapto, one of the founding Grey Knights.)
  • Nero Vipus (Killed on Isstvan III)
  • Vaddon (Killed on Isstvan III by Lucius)
  • Unnamed Pre-Heresy Era Luna Wolf discovered in stasis on a Space Hulk and revived by the Flesh Tearers in M38 (Went berserk after getting an update on what happened before “borrowing” a warp jump capable vessel. Apparently now on a mission to personally put a boot in the ass of every traitorous Son of Horus he can find).
• World Eaters
  • Macer Varren (Recruited into the Knights-Errant, killed on Terra by the Lord of Flies during the Battle of White Mountain)
  • Endryd Haar (Became a Blackshield during the Horus Heresy and also rumored to have been a hybridized-legionnaire with ties to the Thunder Warriors, died while punching the shit out of Abaddon at the Siege of Terra)
  • Tagore (Member of the Outcast Dead, slain on Terra)
  • Subha (Member of the Outcast Dead, slain on Terra)
  • Asubha (Member of the Outcast Dead, slain on Terra)
  • Brother-Captain Skraal (Led a force of 20 loyalist World Eaters. Fought alongside the Space Wolves and Ultramarines against the Word Bearers to weaken their forces during the battle of Calth, slain on the Furious Abyss)
  • Ehrlen (Killed on Isstvan III)
  • Shabran Darr (Killed on Isstvan III)
  • Karnaugar (Killed by his gene-father on Isstvan III with Angron staying at his side as he died out of respect for giving Angron a wound that nearly killed him)
  • Juljak Nul, "The Storm Walker" (killed on Isstvan III during a battle between the loyalist and traitor World Eaters, though not before killing several of the traitors.)
• Thousand Sons
  • Atharva (Executed by Dorn on Terra, though either he or his shade appeared to Magnus after his defeat against Russ)
  • Revuel Arvida (Fused with a fragment of Magnus and took the name Ianius, later the first Supreme Grand Master of the Grey Knights)
  • Omari Anat (captured by the Flesh Tearers, stayed their captive to test whether or not Nassir Amit would go insane and kill him despite ample evidence of his innocence, fate unknown)
  • Mhotep (Captain of the Waning Moon. Fought alongside the Ultramarines against Word Bearers/Chaos to weaken their forces during the battle of Calth, slain on the Furious Abyss)
  • Izzakar Orr (part of the original defense of Prospero, he was slain by the Space Wolves in a warp maze. His soul later helped the Space Wolves in M42 find the maze and rescue 200 Space Wolves who had been trapped since the Burning of Prospero. After reuniting and resurrecting his body, he learned that the attack on Prospero had been ordered by the traitorous Warmaster Horus Lupercal and not the Emperor of Mankind. He later closed the warp gate to the maze in order to prevent Magnus the Red from following the Space Wolves).
  • Phosis T'Kar (part of the original defense of Prospero. Came to his senses and realized he had been warp corrupted during the battle. He then immediately stopped fighting and allowed Constantin Valdor to execute him rather than continue falling to warp mutations/corruption).
  • Magister Sul Kontep (leader of a part of the Fifth Fellowship loyal to the Emperor, who surrendered and were imprisoned in Palace dungeons. Having received a chance for redemption after beggining of Heresy, they fought on the side of the loyalists during the Cthonia campaign).
• Night Lords
  • Fel Zharost (Former Chief Librarian, sent into exile after Nikaea. Recruited into the Knights-Errant. Possibly became Khyron, one of the founding Grey Knights)
  • Kasati Nuon (Aided the Raven Guard in their guerrilla war against the Traitor Legions)
  • Vilnius Malik (Abandoned the Night Lords after becoming disgusted with what they had become. Joined up with the loyalist Alpha Legion warband The REDACTED.)
  • Kellendvar (Originally a traitor that attacked Sotha, during the battle he saved Ultramarine Captain Corvus, from his own Chaos-corrupted brother. He then surrendered himself to be executed, which he was swiftly granted.)
• Word Bearers
  • Kaspian Hecht/Barthusa Narek (Initially drawn to the Knights-Errant. Went rogue and independent of all other factions. Helped Eldrad kill off the Cabal before returning to the Word Bearers secretly with plans to assassinate Lorgar with the Fulgurite leftover from the attempted assassination on Vulkan. Final fate unknown.)
  • Volkhar Wreth (Member of the Crusader Host. Mortally wounded and forcibly bonded with a Daemon on Terra)
  • The Anchorite (Disillusioned Word Bearer who surrendered on Calth and was spared by Guilliman. Entered contemplation while detained and re-embraced the Lectitio Divinitatus, which he had memorized in his youth. Interred in an Contemptor Dreadnought after attempting suicide and secretly taught the contents of the book to a selected few in the Imperial Cult's Ecclesiarchy on Almace. Existence revealed during the Indomitus Crusade and somehow manifests powers of a Living Saint.)
• Alpha Legion
  • Legate Chaitan (Leader of a small fleet of loyalist Alpha Legion vessels. KIA alongside his crew and fleet on the strike cruiser Sigma by Alpharius's manipulation of the Sisypheum's crew while the Primarch was disguised as Shadrak Meduson)
  • Occam the Untrue (Leader of the REDACTED warband. Considers his legion a tool to test the Imperium. Currently interrogating a Deceiver shard bound in a Tesseract Labyrinth after a very costly battle in M42)
  • First Captain Ingo Pech (Fought for the traitors for most of the Heresy, but came to realize that Horus winning would doom the galaxy. Sent to Terra during the Siege to activate hidden XX Legion assets to fight for the Emperor, but was psychically hijacked by Cyrene Valantion to aid in her plans. Left beneath the Palace with a mine clamped to his chest.)

Loyalist Legion Traitors

Traitors? In our Loyalist legions? It’s more likely than you think.

• Raven Guard
  • Alastor Rushal, “The Raven”: Was captured, tortured, and brainwashed by the Night Lords. Had his tongue cut out. Had no official rank but was somehow able to rise to the Kyroptera, the informal council of captains of the Night Lords. Wielded a gravity hammer weapon, at least part of the time, which is usually a World Eaters weapon. Raven Guard in the Night Lords wielding a weapon usually used by World Eaters. Hmmmm...
  • Nathian: Left for dead on Istvaan V. Lorgar found him and converted him to Chaos. He later joined the Emperor’s Children to be a mad doctor.
  • Some of the Terran-born Raven Guard who'd served with the Luna Wolves prior to Corax's rediscovery defected to the Warmaster's side since they felt more at home with Uncle Horus than with Papa Corax, who'd never liked them very much and had exiled most of them to distant crusade fleets. After the war they vanished and were never seen again...supposedly...
• Dark Angels.
  • Luther. This is so complicated it's not worth getting into here, just look at the Dark Angels and Fallen Angels pages.
  • Merir Astelan. One of the first 5,000 Space Marines. Claims to still be loyal to the Emperor, but seems to be mainly out for himself. Responsible for the whole "Lion el'Jonson was sitting out the Heresy to see who would win" meme. Part of one of the factions that formed in Caliban. Again, look at the Fallen Angels page for details.
  • Vortigern. Defected to Horus and after Horus's death was promoted to the Ezekarion, the Black Legion's high command.
• White Scars
  • The Legion almost fell into a civil war until Jaghatai Khan intervened and showed them what the traitors were actually doing. Those who had gone too far were executed and the rest were sent on suicide missions as the Sagyar Mazan, the last of whom died trying to delay Mortarion long enough for him to get blown up by the overloading reactors on Jaghatai's flagship. Ironically, despite this near disaster, the Scars got off pretty lightly given that the stupid parts of the legion who would fall to Chaos lies were most of the casualties by the end of the Heresy. Others that were cut off joined up with the Night Lords during the Thramas Crusade, but sacrificed themselves once they realized their allies were wanton psychopaths and because they were actually loyalists tricked by the Night Lords into believing the Dark Angels were traitors due to the First Legion's isolation, all died in the fighting attacking the Night Lords. One group who stayed loyal to Horus and avoided the almost civil war by immediately leaving almost made it to Terra, but got caught by Nathaniel Garro and Macer Varren and got butchered to the last. Also, the Khan was to fell to Chaos but someone switched his fate with Fulgrim's, with both the Emperor and Malcador surprised by this outcome as it was caused by an unknown third party.
• Iron Hands
  • A company of Iron Hands bearing mixed X and XVI Legion heraldry fought at the Battle of Mezoa, but whatever happened to them after that is unknown.
• Space Wolves
  • Conquest mentions that there may have been an entire Great Company who turned traitor, unlikely as it seems.
• Ultramarines
  • Yes, not even the Smurfs were immune. Consul-Chaplain Ankarion of the 547th Company fell to Chaos early in the Underworld War on Calth and led the Word Bearers in butchering 200,000 civilian refugees who had taken shelter in the Auroran arcology, along with the Ultramarines of his company. When the 4th Destroyer Company of the Nemesis Chapter came to retake the arcology, he skewered their captain on his ritual spear. Said captain dragged himself down the spear so he could stab Ankarion in the throat and trigger a melta bomb, killing them both.

List of suspected chapters

It is important to remember here that mere tactical or heraldic similarities are not enough of an indicator for claiming that a Chapter might have geneseed from one of the nine traitor Primarchs. The cultural influence from recruitment worlds combined with millenia of adaptation and evolution can result in Chapters adopting traits that might echo the matrial ways of the ancient Legiones Astartes, without them bearing any relation to them at all.

The telling signs one must be on the lookout for are first and foremost the "historical" ones, that is to say Chapters whose origins trough can be traced back to the first millenia of imperial history trough things like their deeds or their armory, and then everything that might be "genetic". Because of the genetic link to their Primarchs, geneseed influences the behavior and mindset of a Space Marine. Chapter culture might affect the way in which it is expressed, but it will always manifest to some degree in some way shape or form. For example, the Iron Warriors already had a martyr complex before the discovery of Perturabo, as proven by their actions during the siege of Forge World Incaladion.

Without further ado, let's see what we can find, organised by likeliness;

All But Confirmed:

• Blood Ravens The only known copies of their lineage are currently sealed under the orders of the Ordo Malleus:
  • (Thousand Sons) - All but confirmed. Links to the Thousand Sons Corvidae Cult, high number and heavy usage of Librarians (especially to look into the future to scry enemy's plans like the Corvidae Cult). Just prior to the Fall of Prospero, a psyker named Kallista Eris foresaw the future of the Thousand Sons Corvidae Cult, during which she all but states in incredibly on-the-nose dialogue they will become the Blood Ravens: "The Ravens. I see them too. The lost sons and a Raven of Blood. They cry out for salvation and knowledge, but it is denied!”
  • (Word Bearers) - Speculated, due to operating in Aurelia sub-sector and from the world Cyrene (names associated with the Word Bearers, and an area originally controlled by the Word Bearers during the Great Crusade), as well as a fanatical search for Knowledge (However this is also a trait of the Thousand Sons). Their warcry "Knowledge is power, guard it well", is a direct quote from the Word Bearer First Chaplain Erebus (However this was also a common saying of the Thousand Sons and spoken by the Thousand Sons Revuel Arvida). However, the Thousand Sons connection is so obvious that this connection is a long shot at best.
• Minotaurs:
  • (Old Lore) (World Eaters) - Speculated. Similar battle doctrine, known for being complete berzerkers in battle. However this maybe due to their chimeric geneseed just making them batshit insane.
  • (Revamped Lore) (Iron Warriors) - All but confirmed. They maintain a similar battle doctrine that favours close combat but is now also focused on pragmatic, siege, and attrition warfare tactics (their special rule is a near exact match for the Horus Heresy Iron Warrior's signature special rules). Their Ancient Greek theme also matches up with the Iron Warriors' anatolian/eastern-byzantine/ottoman references, and their gene-seed is mentioned to have extremely high assimilation rates, just like the IVth Legion of old. They even have records of action dating back to M32. Horus Heresy Book 6 "Retirbution" also alludes to the Iron Warriors being the progeniotors of the Minotaurs, as there is described a special unit of Iron Warriors who use heavy hypnoindoctrination and specialized in Counter-Astartes tactics. Edwin Brown, a playtester for the Badab War books, has also stated that the Minotaurs use Iron Warriors gene-seed.
  • Of note: the "new" lore alludes to the possibility of there having been two Chapters called the "Minotaurs", with one being more similar to the old lore descriptions, being part of the 21st Cursed Founding of M36, and references to chimeric gene-seed. Presumably, the old lore Chapter had chimeric World Eater/someone else (possibly Iron Warriors) geneseed and their creation/extinction was to secretly legitimize the "original" Iron Warrior Minotaurs under the direct command of the High Lords of Terra. Or the Imperium's shitty record keeping, combined with the (Iron Warriors) Minotaur chapter being so clandestine, lead to a completely unrelated chapter getting the name "Minotaurs" without knowledge of the name having already been taken.
• Shadow Wolves (Luna Wolves) - Confirmed by creator, but in-canon explanations currently lacking. Despite not being Space Wolves successors (being listed as Imperial Fists), they use tons of Wolfish iconography (no, not that kind). Author/creator of the chapter (Aaron Dembski-Bowden) confirms they were created to represent the concept of loyalist Luna Wolves as a tribute to his wife Katie (who is a fan of the Luna Wolves).
• Silver Skulls (Iron Warriors) - All but confirmed. Suspected to be from the gene-seed of the loyalist Iron Warriors, led by Barabas Dantioch, who helped defend Ultramar. The Silver Skulls are well known for siege warfare (including a rank called "Siege Captain", a rank that had only seen use with the Iron Warriors), masters of building fortifications (their fortress monastery is said to have impressed Rogal Dorn himself), and their Chapter iconography/colors are near identical to pre-Heresy Iron Warriors. Their Chapter records are stated as "lost" so they do not officially know which legion they are founded from, despite being Second Founding. Their gene-seed records on Terra are also locked away beyond even Inquisitional authority. The author of the Silver Skulls series has said they are successors founded from Dantioch's garrison, and were adopted by the Ultramarines (thus making them a case of the Ultramarines Connection). She has also said that their religious aspects are just due to the culture they are recruited from, and Silver Skulls from outside their main recruitment world find these traditions strange and backwards as well.
• Death Eagles (Emperor's Children) - All but confirmed (origin suspected due to records of a Loyalist Blackshield warband from the Emperor's Children's Death Eagles company during the Horus Heresy) while the Chapter's recorded actions stretch back as far as M32.

Extremely Likely:

• Marines Malevolent (Iron Warriors) - Speculated. similar behaviors to Iron Warriors, some similar iconography. Extremely pragmatic, consider one another disposable, complete disregard for causalities or collateral damage, an extreme disdain for weakness, a tendency to challenge other Chapters, and they are pretty entitled little bastards. Like the Minotaurs, they take the wargear of defeated Astartes foes as trophies. They basically are the Minotaurs, if the Minotaurs DIDN'T have the backing of the High Lords. Their veteran's markings are also identical to the Iron Warrior's signature yellow/black hazard stripes. Lastly, but most noteable, their emblem (a winged lightning bolt) was the original emblem used by the IVth Legion, before reuniting with Perturabo.
• Mortifactors (Death Guard) - Speculated. Similar color scheme and iconography to pre-Heresy Death Guard, specialize in night fighting like the Dusk Raiders, similar grim attitude, and have a preference for using Power Scythes (the signature weapon of the Death Guard). This Chapter is also obsessed with death. I mean really obsessed with it. Being a Second Founding Chapter, they could be a prime example of the Ultramarine Connection.
• Iron Snakes (Alpha Legion) - Speculated, officially from Ultramarine stock. Use combat doctrines and show behaviors extremely similar to the Alpha Legion. Similar iconography (a single-headed snake instead of a three-headed hydra). The Iron Snakes also have unusually specific design similarities to said Alpha Legion. Such as their Chapter Master being taller than usual for an Astartes, conceals his face, and has been known to swap identities. They also separate their Chapter into independent cells, just like Alpha Legion harrows. Due to being Second Founding, they are also likely an example of the Ultramarines Connection.

Likely:

• Sons of the Phoenix (Emperor's Children) - Suspected. It's an open secret that Papa Cawl got a hold of uncorrupted III Legion gene-seed). They have similar iconography to the 3rd legion, and their name also possibly references Fulgrim, who was known as the Palatine Phoenix. Known for liking to put on a show when in combat. That being said, the creator of the chapter (Maxime Pastourel) stated that he intended them to be Imperial Fists successors and that the choice of colors was meant to give them an 'ecclesiarchical'. This could be a rare case where the obvious ressemblance is purely coincidental.
• Raptors (Chapter) (Night Lords) - Speculated, only rumored to be Raven Guard. Gene-seed more similar to Night Lords. While they suffer the pale skin/black eyes seen in both Raven Guard and Night Lords, unlike the Raven Guard but just like the Night Lords, they have fully functional Betcher's Glands.
• Mantis Warriors (Alpha Legion) - Speculative, officially from White Scars stock. Like the Alpha Legion they operated from a series of hidden bases spread across the sector of space they were guarding rather than having a single HQ. In-setting they're suspected to be descended from the White Scars, but the source making that claim is a in-setting book written M36 by a High Lord of Terra that has been edited multiple times since then by other High Lords and multiple Inquisitors. Their tactics are more reminicent of the Alpha Legion than the White Scars, with a focus on stealth, ambushes and the like. Their colours (green and yellow) match the Rogue Trader-era AL paintjob, and FW's book mention that the Alpha Legion have been seen fighting in green power armour. They maintain elite cadres of skilled marksmen who assassinate enemy leaders, rather like the 30K-era Headhunter Killteams of the Alpha Legion. Not to mention, the Alpha Legion fought the White Scars multiple times across the Heresy and FW's version of the Chondax campaign has several hundred White Scars on recon missions "disappear mysteriously". Finally, an alternative name for the Mantis Warriors is the "Mantis Legion".

Other Possiblities:

• Brotherhood of a Thousand (Thousand Sons) - Speculated. Minor Chapter with a tiny footnote implying heavy librarius and have a similar name. Use an "M" as their Chapter symbol, similarly to the heraldry of the Pre-Primarch era Thousand Sons.
• Red Scorpions (Emperor's Children) - Speculated. Have an obsession with the "perfection" of their gene-seed, to the point of being outright hostile to any they see as "imperfect". Said geneseed also happens to have a mutation rate inferior to even that of the Ultramarines and Dark Angels (purest among the loyalist legions), but the Scorpions refuse to surrender any samples to the Inquisition. There is also the fact that their armories are stacked with old-patterns of tech, such as Deimos-pattern predators, Proteus-pattern Land Raiders, and MKIV dreadnoughts; which unlike the Minotaurs, cannot be explained by High Lords favoritism. Unlike the Death Eagles or Carcharadons though, there are no records of them existing as a Blackshield force during the Heresy. The Chapter only enters Imperial records for the first time until M34, well after The Howling, and the 4th Black Crusade.
• Storm Wardens (World Eaters) - Speculated. Similar preferred battle doctrine and color scheme to pre-Heresy War Hounds (armored assaults combined with close-combat infantry, blue armor with white pauldrons) (or maybe just like most Ultramarines successors). Records of their origins have been "deliberately" lost or destroyed (like the Lost Legions'). Additionally their homeworld is sanctioned against contacting the rest of the Imperium (not simply purged like a Chaos hive would have been) and is basically sealed off from the outside world. Though they are frankly more likely to be Iron Hands (the previous token "Scottish chapter"; also previously named "Storm Walkers") successors due to their "special friendship" with the AdMech and fondness for hiding inside metal bawxes. Or, like, possibly ultramarines like basically any other blue boys who use standard marine battle tactics of mechanized infantry, but they may just be the "Ultramarine Connection" counterpart to the Emperor's Spears' actual celtic sons of Guilliman.
• Fire Angels (Word Bearers) - Speculated. Highly similar behaviors and doctrine. Strict following of whatever rules they are given? Check. Do they follow the Codex to the letter? Check. Highly religious? Double check. Don't like Guilliman? You can bet that's a check. The combination of all of these traits makes them essentially loyalist Word Bearers in all but name... but that's as far as the similarities go. They aren't the only highly religious Astartes Chapter, nor are they the only Chapter to adhere so strictly to the Codex (especially among Ultramarines), or the only "Ultramarines" successors to not like Guilliman (see the Mortifactors above). There isn't any evidence hinting that their behavior is the result of genetics and not the cultural changes undergone by the Imperium in 10 000 years, especially considering that they are a Chapter that wasn't founded until very recently. However, much like the Minotaurs, there are noted records hinting that theirs is not the first incarnation of the Fire Angels Chapter.
• Covenant of Fire (Word Bearers) - Speculated. While officially listed as "Salamanders", the Covenant of Fire could be Word Bearers who follow the Promethean Cult instead of worshiping the Emperor or Chaos. Their iconography is very similar to the pre-Heresy XVII Legion, and their attitudes and behaviors are also extremely similar to the Word Bearers, most specifically their "Ashen Circle" units. Seems Cawl could have run out of fucks to give and is now just seeing how far he can push it before Guilliman catches on.
• Sons of Antaeus (Death Guard) - Speculated. Similar genetic traits included an unnatural hardy stock.
• Death Spectres (Death Guard) - Speculated. Similar name, as well as similar iconography to the pre-heresy Death Guard. Their homeworld of "Occuludas" is also the last known location of Crysos Morturg, a loyalist member of the Death Guard, as well as his "108th Independent Company". They also prefer using Power Scythes in combat, a preferred weapon of the Death Guard. Like the 108th Independent Company, the Death Spectres have been tasked with protecting the Imperium from threats originating from the nearby Ghoul Stars. There is a lesser known background book that makes them out to be Raven Guard, but much of the book has been ignored, contradicted, or outright retconned since then, so it's likely their chapter origins have been too.
• Shadow Haunters (Night Lords) - Speculated. While they are listed as Raven Guard from the 19th Founding, their name harkens back to the title taken by the Primarch of the Night Lords: Night Haunter. They specialize in terror tactics and they suffer from a geneflaw which causes them to have horrific visions, similar to the geneflaw suffered by the Night Lords. Granted, this information comes from the chapter's creator's self-made Index, and not from an officially published source.

Special Cases:

• Grey Knights (Founders were initially drawn from the Knights-Errant) - Outright stated, although only from their history. While several Astartes from Traitor Legions were among the founders of the Grey Knights, the Grey Knights believe their gene-seed is derived from the prime template of the Emperor, though whether that's true is a point of contention among the fanbase (despite it being repeatedly and clearly stated in any and all Grey Knight's fluff, it's still unclear just HOW Big E made it).
• Exorcists (Official Imperial Fist successors. Classified and speculated Successors of the Grey Knights in old lore...alternatively Word Bearers) - The link to the Grey Knights is iffy since only due to a small mention by an Inquisitor, and the Grey Knights are notoriously against ANYONE messing with their gene-seed. However, the Exorcists colors, scripture written on their armour, and nature as anti-demon specialists is almost 1 to 1 with Heresy-era Word Bearers. It is not completely impossible since they are from the Dark Founding.
• Carcharodons (Traitor Raven Guard/Night Lords/World Eaters) - Speculated. Physical traits (black eyes, pale skin) similar to both Raven Guard and Night Lords. A gene scan of a dead Carcharadon revealed markers for Raven Guard. They also suffer a Battle Rage they called "The Blindness" which manifests exactly like the Raven Guard's "Ash Blindness". They often have a black and white morality, focus on shock and fear tactics combined with stealth, and their preferred tactic is to brutally and horrifically annihilate the target population (be they enemy or civilian), but leave a few survivors to go tell everyone else about what happened. Likewise, a Night Lord officer noted that the Carcharodons seems to be a mirror of their own legion, which the original XIXth legion were known to have been. These further indicate they are both of the XIXth Legion, specifically the Terran born XIXth Legion under the command of their original legion commander: Shade Lord "Arkhas Fal", as the Carcharodons share the title for their leader: "Shade Lord". Notably, the Ashen Claws (a nominally loyalist renegade warband form of a different group of Terran-born XIXth Legion who had been exiled by Corax before the Heresy) have flatly called the Carcharodons "traitor-born", as it was most likely Arkhas Fal's Predation Fleet that joined Horus's side in the war. At the very least, it was likely the Carcharadons are who joined Horus, as he gave them the name "Pale Nomads", a title now used by the chief librarian of the Carcharadons. Of course, there’s also the possibility that they could have a chimera geneseed…which may not have been chimeric originally.
• Ashen Claws (Night Lords/Renegade Raven Guard) - Note: While technically not an officially listed chapter, they have recently begun to fight for the Imperium again. Similar to the Carcharadons, they were founded by an exiled predation fleet of the Raven Guard. This time, the fleet lead by Shade Captain Nerat Kirine, the fleet exiled after the Battle of Gate-42. During the heresy, the fleet returned and declared they fought for neither Horus nor the Emperor. Instead they fought for themselves. They raided the Nostromo Sector, looting war gear as well as stealing the geneseed from the Night Lords they slew.
• Angry Marines (World Eaters) - They can say whatever they want about who they think their Primarch is, but the Inquisition still suspects Angron.

The Ultramarine's Second Founding Connection and Lost Legion Loyalists

The Ultramarines are confirmed to have harboured loyalist members of Traitor Legions after the Heresy, rebranding them as their own to obfuscate the newcomer's past (as for why they would do such a thing, Roboute Guilliman himself is characterized as someone who abhorred the waste of any human life, Astartes or otherwise). Guilliman also needed all the manpower he could get when he thought the rest of the Imperium was destroyed during the era of Imperium Secundus, so giving even legionnaires from suspect sources a chance to prove themselves was a practical solution, for which in the end he was not disappointed. The penning of the Codex Astartes and the shattering of the Legions into individual Chapters would also have provided the perfect opportunity to separate the loyalist Traitor Legion elements back into separate formations (both to work more as they were intended, and to make it easier to exterminate any of them that decided to betray the Imperium at a later time). Furthermore, it is confirmed that after [REDACTED] that led to the deaths of the Missing Primarchs happened, some/most Astartes of the II and XI Legions were mind-wiped by Malcador the Sigilite into thinking they had always been part of the Ultramarines and Imperial Fists (or possibly other legions), as to not waste useful Astartes assets. This act was backed up by at least Dorn and Guilliman voluntarily agreeing to have their memories of what really happened suppressed beyond even a Primarch's ability to recall. So, the speculation regarding the Soul Drinkers, Valedictors, Rainbow Warriors and Storm Wardens (along with the tinfoil hat addition Black Templars) being lost legion loyalists might have some truth to it after all.

Of the Chapters listed above, the Chapters that are suspected of this connection are:

• Silver Skulls - Iron Warriors
• Mortifactors - Death Guard
• Iron Snakes - Alpha Legion

Sources

Forge World Horus Heresy Rulebooks

• Betrayal
• Retribution

Horus Heresy Novels

• Flight of the Eisenstein
• Pharos
• Vengeful Spirit
• Corax
  • Weregeld

Other

• How to Paint Space Marines, pg. 86
• Apocalypse by Josh Reynolds