Omegon
The twin brother of Alpharius; collectively, they are the Primarchs of the Alpha Legion. People know less about this guy than they do about Alpharius, so that's pretty much less than jack squat. In fact, people know so little about this guy and his brother that it isn't even known if they are one person or two. This is leveraged by the fact that everyone is either one of these guys or the other, including themselves.
If you don't know anybody who is secretly Alpharius or Omegon, you probably are him (or the other one (or both (or neither (or all at the same time)))).
Conspiracies within Conspiracies (as usual)
Alpharius/Omegon despite being twin primarchs have been shown to be at odds with each other, both helping and hindering the Horus Heresy to varying degrees. Therefore it is possible that one is a loyalist while the other is a traitor. Consider the following evidence:
Undermining Horus and the Traitor cause
The Alpha Legion's first victory in the Horus Heresy was claiming the Forge World of Paramar, which was left in such a state that Horus had to garrison the world with substantial forces earmarked for warzones elsewhere just to maintain control over it. When Horus called Alpharius out on this bullshit, the encrypted response was "The Devil is not Mocked" which is an oblique reference to a proverb (the full saying of which goes "When evil faces evil, the Devil is not mocked"); the moral being just because they're both evil doesn't mean they're on the same side, and that a major evil badass won't automatically consider a smaller, also evil upstart his equal.
When the White Scars eventually left the Chondax system, an Alpha Legion "fleet" (actually made up of several "fake" battlecruisers with no real teeth) arrived to hold them in position long enough to receive orders from Rogal Dorn to return to Terra. Mortarion has no idea what that was meant to achieve (the mission was apparently to give them misinformation about Prospero and con them into helping a different, non-toothless Alpha fleet to wipe out the Space Wolves) and comments that Alpharius will end up "throttling" himself with all his schemes. Given that the other fleet did its best to wipe out the Wolves, it suggests two Primarchs working two different goals.
Someone in the Alpha Legion saved Corax and the survivors of Istvaan from the World Eaters. If Corax died, the Raven Guard would have been finished and out of the war for good (much like the Salamanders were) instead they went on to rebuild and rejoin the war towards the end and still have enough marines left over to split into successor chapters.
- Also the Raven Guard being saved allowed Alpha Legion Infiltrators to steal Primarch Gene Tech which they were forewarned of by the Cabal.
Alpharius vs Omegon
Alpharius/Omegon sided with Horus despite being explicitly told by The Cabal prior to the Heresy that if Horus won it would mean the end of the human race but the ultimate victory against Chaos. While if the Emperor won it would mean the decay and eventual death of the galaxy, but humanity survives. A Catch 22 with two undesirable outcomes, but hey, the Alpha Legion has two Primarchs, go figure?
- By the time of Vulkan Lives, the Cabal have apparently written off their "Alpharius Gambit" as a failure, despite from all appearances that the Alpha Legion joined Horus just like they wanted.
Omegon discretely plotted against Alpharius to destroy an Alpha Legion facility which was causing warp storms and preventing the White Scars from returning to Terra. If this facility was not destroyed the White Scars would not have been present during the final battle.
- Evidence of one brother conspiring against the other? Just like the rest of the Horus Heresy. Just because they are twins it doesn't mean they are always of the same mindset.
In Seventh Serpent by Graham McNeill, a sizable fleet of Alpha Legion Loyalists was preparing to join the war effort, until Traitor-Alpharius manipulates the Shattered Legion Loyalists into destroying them, under the ironic pretext of assassinating "Alpharius".
- Alpharius refers to himself here as "the one and only", which at a stretch might be a subtle hint that he has done away with his twin. As the existence of his twin is a monumental secret that there would be no need to draw attention to the fact there is only one of him, short of ego that is.
- The Loyalist Alpha Legionnaires queried "which one of them is it?" upon realising that other Alpha Legionnaires were in orbit. Then later determined "You're one of his" as if the distinction between the two Primarchs mattered at this point.
Formation of Malcador's Grey Knights
Despite the Alpha Legion stealing the ORIGINAL genetic template of the Primarchs, they handed false information to Horus which would never have resulted in anything useful.
The said stolen genetic information was never utilized by the traitor Alpha Legion either, even after 10,000 years, even when they could have been producing "super" space marines! But hey? aren't Grey Knights an "original" space marine chapter with NO known Primarch? with psychic powers? AND an unusual resistance to corruption? And supposedly improved genetic template?
- Also 7th edition Grey Knights Codex hints that it has traits that none of the Legions Gene-Seed has (such as making the purer/more resilient to Warp Corruption (read Castellan Crowe's entry) which would mean that they are an entirely new strain of Gene-Seed and someone would have needed the knowledge/data to be able to create it.
Is Omegon Janus?
Not likely, considering that one of the Alpharius twins was killed by Rogal Dorn. "Omegon" then reluctantly had to take the role of Alpharius full time. Though that would obviously not stop him and his legion from pulling body double switching and continuing the ruse that anybody could be Alpharius. Yet if we take the Ultramarines account at face value, then Guilliman killed Omegon-Alpharius which would have meant the end of both twins.
"Ianius" would arise separately as a fragment of Magnus fused with one of his last remaining loyalist Thousand Sons and is a very strong candidate for being "Janus".
That does not stop the speculation of what Omegon might have been if his plans had continued, or if Guilliman did not actually kill him and Omegon changed his name and went into hiding elsewhere.
He kept a hidden suit of armour on board his battle barge in addition to his "normal" legionary armour, one which he had planned a hidden purpose to "strike unexpectedly like a serpent". This other suit was plain and unadorned... (unpainted power armour is grey, also it's what Malcador's Knight-Errant wore.)
The first Grand Master of the Grey Knights was named "Janus". (the Roman god with two faces of "beginnings & endings" i.e.: Alpha & Omega)
Furthermore, in Mortarion's Heart Mortarion claimed that he knew "Janus" this could imply that Janus was either a Death Guard or one of his brother Primarchs (i.e Garro or Omegon)
Additionally, during the the Primarchs anthology book we get another hint that this might actually be the case. During the Iron Hands portion of the book while Ferrus Manus is tripping on Eldar psychic-fuckery he walks through a hallway filled with statues. While the book doesn't describe every statue in detail it becomes pretty clear quickly that each statue represents a Primarch. Ferrus Manus confirms this himself when he realizes one of the statues is supposed to be Fulgrim. Why does this shit matter? Because one of the statues that is gone into with some depth is the one that Ferrus recognized as being the Roman god Janus. Oh boy. The book makes it clear that this statue is supposed to represent Alpharius and Omegon (I mean, what other Primarch could you describe as having "twin-faces"?). So in short, in a hallway of statues that represent the Primarchs the one that seems to represent Alpharius Omegon, is straight up called Janus. Holy shit.
Let's just pray to the Emperor that this is not all just coincidence.
Which Alpha Legionaire Are You?
Black Library has an Alpha Legion quiz on their site. About half the answers seem to lead to Alpharius, and the other half lead to Omegon (fitting for the two Primarchs). If you get "I am Alpharius" for your result, there's a description of him that basically encompasses all answers (but does tellingly say "You are absolutely loyal to your legion") whereas Omegon's description says he answers to no one in pursuit of his mysterious goals and will kill anyone in the way... Actually, we kind of already knew that, but at least this confirms it.)
Tabletop
| WS | BS | S | T | W | I | A | Ld | Sv | |
|---|---|---|---|---|---|---|---|---|---|
| Omegon: | 7 | 7 | 6 | 6 | 6 | 6 | 5 | 10 | 2+/4++, or is it? |
While not a monster of close combat like other primarchs, Omegon gives candy to his army while fucking whith the opponent's like no one but he can (That being said, even the worst of combat primarchs will whomp just about anything else). Sire of the Alpha Legion grants all Alpha Legion units Preferred Enemy (Everything) and gives Omegon Preferred Enemy (Everything), Counter attack, Move through Cover, Scout, and Crusader- but not right away. You see, after both sides deploy he can opt out of deploying normally or via deep striking and choose to hide in any Alpha Legion Infantry unit which is part of the Primary Detachment, disguised as a rank-and-file model, but can reveal himself starting at turn 2 (and does so automatically at turn 5) replacing a model and gaining all of the above buffs. If the unit he's hiding in is "falling back" when he reveals himself, it auto-rallies, and if it's destroyed while he's still hiding (or if it is in a Transport and there is no space for him), he just goes into reserves (you must write which unit is hiding Omegon, but if you're a true Alpha legionnaire you'd have 6 different notes, none of them with your actual handwriting style). Be careful though: remember that he is Bulky and that he can only be revealed at the beginning of your own turn, so if he is hiding in a transport at full capacity he could be impossible to place (for example in a ten man Breacher squad inside a Rhino) unless you already disembarked the Squad on the previous turn, so plan accordingly. He can also Seize the Initiative on a 4+, give units arriving via Outflank a +D3 movement bonus and, best of all, if an enemy unit is successfully brought in from reserves and your army has an identical version of that same type also in reserve, roll a D6: on a 4+, the opponent brings in your unit instead. Combine him with his Legion RoW to almost guarantee going first and almost completely negate your enemy's reserves. On defense,The Pythian Scales gives him 2+ armour save and 4+ invulnerable save and also ignores the effects of Poison and Fleshbane. Not bad. His weapon, The Pale Spear doesn't boost his Strength, but it does have AP1, Armourbane, and Instant Death along with Two-Handed. He also has the following equipment: Venom Spheres, Master-Crafted Plasma Blaster, Cognis Signum, Nuncio Vox and Camelioline, making him quite the force multiplier and perfectly capable to adapt to a variety of circumstances, like he should be.
| The Primarchs of the Space Marine Legions |
|---|
 Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan |
 Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo |