Siege of Terra
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| Siege of Terra | |
| |
| Date | 0014.M31 |
| Scale | Planetary |
| Theatre | Horus Heresy |
| Status | Pyrrhic Loyalist Victory |
| Belligerents | |
| Traitor Legions | Imperium of Man |
| Commanders and Leaders | |
| Horus, Angron, Mortarion, Angron, Fulgrim, Perturabo, Magnus the Red, Zardu Layak, Kelbor Hal | The Emperor, Sanguinius, Rogal Dorn, Jaghatai Khan, Vulkan, Malcador the Sigillite, Constantine Valdor |
| Strength | |
| Sons of Horus, Death Guard, World Eaters, Emperor's Children, Thousand Sons, Iron Warriors, 5000 Word Bearers, Night Lords, Traitor Army forces, Dark Mechanicum, Traitor Titan legions, daemons | Imperial Fists, Blood Angels, White Scars, Adeptus Custodes, Sisters of Silence, Knights-Errant, Imperial Army, three Titan Legions |
| Losses | |
| Massive Heretic Astartes casualties, massive Traitor Army losses, massive Traitor Titan losses, massive Dark Mechanicum losses. Horus slain. | Massive military and civilian losses. Malcador the Sigillite slain. Sanguinius slain. Emperor mortally wounded and interred into Golden Throne. |
| Outcome | |
| Traitors driven from Terra and into the Eye of Terror. Death of Horus and crippling of the Emperor. Great Scouring Begins | |
"He waits no longer. It begins now."
- – Sanguinius on the 13th of Secundus, 014.M31
The Siege of Terra was the end of the Horus Heresy in Warhammer 40,000. If the Horus Heresy can be considered the most important series of events in the 40k universe(*cough War in Heaven), then the Siege of Terra itself could be considered the single most important event. It is also possibly the most fucking awesome event; Brother fighting brother, the mighty guns of Titans blowing fortifications to shreds, Imperial Army soldiers leading charges against the traitorous forces even though they know it's suicide and Ollanius Pius
It was Horus's big attempt to off his daddy and so be the true Emperor of the galaxy (for Chaos of course!). He brought a load of his traitor legions, millions of corrupt Imperial army personnel and mutants, the part of the Mechanicus that had gone over to his side and a whole load of daemons to boot. On his side the Emperor had three legions, his custodians and the loyal imperial army regiments of Terra and you know what? The Emperor went and won anyway (granted it was because the Emperor offed Horus before his legions could crack the Imperial palace but still, victory for the home team!).
The Solar War
Dorn began fortifying Terra immediately after getting word of the Heresy, knowing that it would always be Horus's eventual goal. Despite being removed from the larger battles of the Heresy, the Solar System was touched by the conflict, with Mars rebelling and numerous sleeper agents and cults trying to destablise the Throne World. Despite this, Dorn managed to do the best he could, turning Terra into the most heavily fortified system in the Imperium. He even managed to blunt part of the traitor advance at the Beta Garmon cluster before getting ready for the final rumble they had known was coming.
Terra was unique in that it that it had two artificial mandeville points inside the the Solar system itself, created during the Dark Age of technology. Dorn fortified the likely approaches from the outer edge of the system and built up huge defences around the two internal jump points. Infiltrators and covert operatives sabotaged loyalist assets across the system. The Iron Warriors were the first Astartes into the breach, sending huge up-armoured Space Hulks as fireships to wear down the defenses, before sending their main fleet through to engage the combined Fists and Scars fleets. The inner system conflict went on for a bit, with the loyalists managing to hold out enough to slow down the advance, at least for a little while.
However, under Magnus's direction, the traitors turned the Shrine of Unity comet into a vast warp gate that allowed Horus, Angron, and Fulgrim's fleets to jump right past most of the rings of defense Dorn had come up with. On the Phalanx, Dorn was preoccupied with a daemon incursion and could do little to stop the huge fleets that were now mobbing for Terra. The Martian traitors, free from the blockade that had hemmed them in for years, joined up with Horus. The Solar War had been lost barely after it had begun. The rest of the Loyalist fleets, knowing they could never hope to fight even a fraction of the vast traitor armada, regrouped on the edge of the system, along with the Phalanx, waiting for the moment they could make an effective strike against Horus. There were early plans for the Emperor to be evacuated to the Phalanx and escape Terra, but these were made by people unaware of what Big E was doing in the basement of the palace.
The Siege Begins
"Father! I have come for you!"
- – Angron upon making planetfall, 15 of Quartus, 014.M31
With orbit uncontested, most of the armada held in orbit above the palace and began bombarding the Aegis, the vast shield network protecting the entire palace. Unlike regular void shields, the Aegis consisted of multiple overlapping layers of shields that individually regenerated as fast as they could be depleted by bombardment. On the ground, the palace was protected by colossal networks of walls and bastions, static defences, and vast numbers of Imperial Army units bolstered by hordes of press-ganged conscripts. Unknown to almost everyone, the Inner Palace was also protected by a psychic ward generated by the Emperor that would royally fuck up any daemon that set foot near it, daemon primarchs included.
The rest of Terra wasn't so lucky. Barring a few isolated holdouts, the rest of Terra was virtually defenceless. It should be noted that if the goal was to destroy Terra wholesale, it could have been easily accomplished by Exterminatus level weaponry. Perturabo, as the only non Chaos-ifed primarch, grew increasingly angry at what he saw as an irrational and wasteful goal. But Horus was insistent that the Emperor had to be slain in person. So the Palace had to be destroyed the old fashioned way. In all fairness, one must also ask if exterminatus was even possible when a being like the Emperor was on Terra, to say nothing of the void shields and defenses on Terra itself. The Daemon Primarchs were kept in orbit, safe from the Emperor's wards, although this meant that Angron had to be imprisoned in an almost impenetrable maze to stop him from Leeroy Jenkinsing the whole thing.
Hordes of mutants, beastmen, cultists, and traitor army units were thrown at the conventional defenses. Entire wings of aircraft dueled above the Palace. Precision bombardments gradually weakened minute sections of the Aegis long enough for bombers to get through and destroy the projectors. The Dark Mechanicum landed siege camps at 8 points around the palace - partly to surround the palace but also to act as the focus for a ritual that would enable the warp to take a foothold on the surface of the Throneworld. The Astartes were held in reserve on both sides whilst their more conventional forces softened each other up. The Death Guard were the first traitor Astartes to land on Terra, with the Khan and the White Scars riding forth on jetbikes and aircraft to meet them and wreck the Dark Mechanicum's siege camps. The Night Lords were the first Astartes to breach the walls of the Palace, albeit in small numbers; this attack also cost them their de facto commander, Gendor Skraivok. Sanguinius himself descended to help the mortal forces, acting as both force multiplier, decoy, and morale booster.
Battle for the Lion's Gate
While the Death Guard, Emperor's Children, and World Eaters each hammered away at a different section of the Palace walls, the traitors' first major effort at cracking the Palace itself was aimed at the Lion's Gate spaceport, the largest and tallest spaceport on Terra. It reached so high into the atmosphere that voidcraft could dock at its upper levels, meaning that the Traitor forces could more easily shuttle in reinforcements and materiel if they captured it. Horus tasked the Iron Warriors with taking the Gate, and in turn Perturabo assigned Warsmith Kroeger to lead the assault, under the logic that Dorn would be expecting Pert to command such an important offensive personally and wouldn't be expecting whatever plans Kroeger came up with. Dorn, meanwhile, assigned Seneschal Fafnir Rann to lead the defense of the spaceport, rather than First Captain Sigismund, since he was still angry with Sigismund for listening to Euphrati Keeler instead of obeying his orders. Kroeger went straight for the throat, as it were, launching a massive combined-arms assault directly on the port with backup from the World Eaters and Emperor's Children, though the latter quickly got bored and left after taking a bunch of prisoners for unspecified purposes. Though the Imperial Fists held off the initial attack, Warsmith Forrix and a thousand Iron Warriors managed to infiltrate the Gate by using renegade Imperial Army units as literal meatshields. To aid the attack, the Dark Mechanicum inserted a technophagic virus into the spaceport's systems, and Zardu Layak, Abaddon, and Typhus performed a Nurglite ritual to infiltrate the Great Unclean One Cor'bax Utterblight behind the Emperor's psychic wards.
The Fists drove back several consecutive assaults from the Iron Warriors and World Eaters, but the technophage was screwing their sensors and comms all to hell and gone, seriously complicating efforts to coordinate the defense, and Forrix and his infiltrators were tying up troops that were desperately needed elsewhere. Rann finally called Dorn for backup, and Dorn scraped up an additional three thousand Fists, which were literally all the troops he could spare at that point. Eventually, despite Rann's best efforts, the balance inevitably tipped in the traitors' favor, and Dorn arrived on scene just in time to order a general withdrawal from the spaceport to the inner defenses, though not before he killed Zardu Layak after a brief duel. With the Gate firmly in traitor hands, Perturabo started unloading Titans and consolidating his position.
Meanwhile, Euphrati Keeler and the Custodian Amon Tauromachian had been tapped by Malcador to investigate strange apparitions occurring behind the Palace walls. They eventually deduced that this was a daemon exploiting the faith of Imperial cultists to manifest itself inside the Emperor's psychic defenses, and focused on a cult called the Lightbearers, which turned out to have been deceived into worshipping Nurgle instead of the Emperor. After Cor'bax had used the Lightbearers to physically manifest himself, Amon, Euphrati, and Malcador teamed up to slay the daemon. When Amon suggested that they should purge the rest of the Emperor's worshippers to prevent another such incident, Malcador answered that he would continue to let them exist until the Emperor himself said otherwise, in the hopes that he could weaponize their faith against the Chaos gods.
Saturnine
 This page is in need of cleanup. Srsly. It's a fucking mess. |
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So, remember Sindermann? Well, the book opens with a nearly-suicidal Sindermann coming across Dorn in (one of the few remaining) Palace gardens. They talk, and this leads to an interesting exchange where Sinderman explains to Dorn that what he [Dorn] is feeling is fear. But given that he's a primarch, fear seems to instead manifest in a more clinical, detached way, and seems to instead be stressful rather than debilitating. Dorn says he had many fears, but then asks Sindermann not to tell Guilliman. Dorn adds that he doubts that even a 1000 meter fall would kill a Primarch. Dorn also mentions that he greatly dislikes censoring Keeler, but ultimately has no choice under the circumstances given that she is regarded as a real security risk. To be fair, as the previous novel makes clear, that concern is not exactly unfounded. The palace and its crest have shifted 8cm west from all the artillery fire since the siege began. Sanguinius believes that Abaddon is the greatest First Captain out of the Legions, better than Raldoron, Sigismund and all the rest.
Meanwhile, Abaddon thinks his mind might be the equal of Perturabo. Spoilers: big surprise, turns out that he was wrong... Like very wrong indeed. While Abaddon respects how Pert is orchestrating the siege, he has distain for how detached he is. He argued that the siege would be won with a blade, the way the Crusade had been, not with a button that belonged solely to Pert or Horus. Showing that Abaddon can be nostalgic. Aximand says in disbelief to Abaddon that Perturabo actually looks happy... Happy as a "grox in shit".
Sanguinius fights off three Warhound titans, while single-handedly killing the Warlord they were escorting with his Blade Encarmine and the Spear of Telesto. As the three smaller titans lock weapons on him, he challenges them, like a total badass, "Try if you like. Shall we continue?" They then ran away. Some then say the titans ran from a titan-killer weapon they had detected. But everyone who had seen it knew that it was the work of the Great Angel who sent them running. Dorn discovers a crack in the Saturnine Wall thanks to Sinderman; A crack which Pert will surely use to ram a chainsword up Dorn's ass if he's not careful. If Saturnine falls, then the way into the Inner Palace is open. To re-secure the wall, a ruse-show-sacrifice must be employed. Dorn only has enough forces to defend three key areas out of the four options, so he sacrifices the Eternity Wall Space Port. Big-E refuses to leave Terra, making this the best option. But losing it will reduce the holdout period by four months, and would allow further traitor forces to land. Furthermore, any defenders of the port will die. Amongst the defenders of the port are Cambra Diaz of the Imperial Fists, Vigil Commander Jenetia Krole of the Sisters of Silence and Shiban Khan of the White Scars. Krole had been used as the null-guard of the High Council, now she will use her Blank-Null powers to protect the port from daemons, since any daemon killed by the Sisters of Silence isn't merely banished, but killed permanently. Dorn then has Sindermann set up a proto-Inquisition of former Remberancers. Giving them the ability to interrogate people, that way they can record the events of the siege proper. Though some of this information will be buried because heresy.
Abaddon convinces Pert that Saturnine should be their target, appealing to a clean victory instead of one tainted by the use of daemons. A proper Marine and Primarch vs Marine and Primarch fight. In his pitch, Abaddon alters between stroking Perturabo's ego and goading him. Perturabo eventually agrees, but does still kick Abaddon's ass a bit for good measure, who in turn mostly takes it in stride and just brushes it off. Fulgrim takes quite a liking to Abaddon's plan and volunteers the ENTIRE Emperor's Children Legion to the attack, and will be taking part as a distraction from Abaddon's spearthrust upon Saturnine. Later on, Ahriman is also taunting the now daemon-primarch Mortarion. Ahriman starts summoning daemons when Magnus arrives to comfort Mortarion. Wait what?
One reason why Loken refused to become a Grey Knight was because he was afraid of having psyker powers and fears what it will cost him. Abaddon also doesn't tell Horus about the Saturnine operation, not trusting him. All the more so after learning that Horus has been going increasingly Chaos-senile; inexplicably asking for random trinkets to be retrieved for him, but later having no memory of doing so, or repeatedly calling Argonis, his new equerry, "Maloghurst," the name of his now-deceased previous equerry, for example. How embarrassing. Argonis tells Abaddon about all this, and Abaddon seems to share the same concerns even more than he's letting on. Keeler becomes a member of the proto-Inquisition, but is confined to her prison facility, and so interviews some of the inmates. Accompanied by the Custodian Amon, she interviews a proto-techpriest who has lived for 5000 years, because science, and who has devised the idea of a Primarch/Space Marine killer-poison. Also, Sanguinius is afraid of Pert, claiming him to be the greatest enemy during the Siege. Only comforted by the fact he has Dorn on his side. He would also be more comfortable fighting Horus on the Vengeful Spirit than fighting Pert at range. Zephon, from Master of Mankind, saves a mortal remeberancer, taking an artillery shell to the chest to save her. He may be dead, or just in stasis, or maybe he was given to Arkhan Land. Hard to say. Arkhan Land is now the one working on Saturnine Wall.
The Khan is defending the Colossi Gate from the Death Guard, when 8 warhound-sized daemons appeared, sending fear through him. John Grammaticus appears, meeting a women named Erda. Erda requires her own article due to the sheer nature of the info surrounding her and the myriad implications, but the following is a quick summary of her introduction. Erda explains the nature of the perpetuals, their background, and how it led to Big E's plans (Big E is also a perpetual). She lists one of the Emperor's names as Neoth. She explains her scathing disapproval of Big E's plans and arrogance. Most important however is the revelation that SHE was responsible for scattering the Primarchs, not Chaos. Ohhhh deaaar. There is honestly some interesting stuff revealed here, but by Christ does it seem sloppy. As already stated, Erda requires her own article. Later on, Oll Piers, not Oll Pius, is being interviewed by a proto-Inquisitor. In short, he dies defending a flag with Big-E's face on it from Angron, starting the legends of Ollanius Pius.
Another change in the lore is that Horus ACTUALLY WON THE SIEGE AND THEREFORE THE HEREST THE FIRST TIME AROUND! John Grammaticus' first trip to Terra ended up being Six Months Too Late. While, the book never explicitly shows what he saw he states that it was bad. He only met Erda in his second trip. (This in the only known instance of time-travel having huge effect on the lore. Despite having time-travel powers, neither Eldrad nor Orikan have used them against Chaos yet John was able to rewrite the outcome of the Siege)
At the Eternity Wall Port, Angron offers the defenders a chance to surrender. In kind, they answer him by shooting him with every cannon, bolter, las-gun and brick they had available, turning him into a red pulp. But as Khorne wills, he is reborn. Moving on, White Scar Stormseers counter Thousand Sons daemon-summoning. This then makes Ahriman and Magus depart. It is noted that Custodes, and Valdor in particular can cause real, lasting harm to daemons.
Abaddon and the rest of the Mournival, along with his strike force finally arrive at Saturnine. They are met by a series of kill-teams made up of veteran Blood Angels, Imperial Fists, knights errant and black shield legionaries. Tybalt Marr, who killed Shadrak Meduson, is the first to arrive and so his unit is also the first to be ambushed. Whoops. Getting shot up badly, Loken puts his bolter in Marr's mouth before pulling the trigger. Garro beheads Falkus Kibre. Lev Goshen's drill gets stuck, and then swamped in liquid concrete designed by Arkhan Land, meaning Goshen and his men die a long, drawn-out, embarrassing death. Garro, a Blood Angels terminator host captain and Haar, the Riven Hound, almost kill Abaddon. The Blood Angel dies first, Haar almost powerfists Abaddon to death, but seven mortal wounds end the Riven Hound. Weighed down by Haar's body, Abaddon can not defend against Garro. Near death, he thanks Perturabo for the glorious fight. But just as Garro is about to kill him, Abaddon is teleported away. Crying, Abaddon begs the techpriests to send him back to the fight. It's never stated outright, but he presumably lost his arms in the return teleport trip.
Loken dueled Tormaggedon, and the possessed body proved to be a challenge, but he finally beats it with a stab to the heart with Rubio's gladius. Loken goes on to impale and eviscerate Little Horus Aximand on his chainsword, holding him aloft before then revving it up. He tortures Little Horus whose last few minutes are a drawn out, agonizing scream that overlap with the revving of Loken's chainsword. With Aximand EXTREMELY dead, Loken kicks him off of his chainsword and then takes Aximand's sword for himself. Meanwhile, Fulgrim starts taunting a wounded Imperial Fist, then, suddenly, Sigismund arrived. Sigismund barrelled into him before hacking at him, actually causing pain and wounds against the daemon-primarch. After being kicked away, he then plunged his powersword into Fulgrims's thigh, eliciting a shrike from Fulgrim, who then tried to choke Sigismund to death. Using his wrist and pommel chains, the Templar pulls his power sword out of Fulgrim's thigh and then cuts his face. Dorn (followed by Maximus Thane) then appears to save the day, giving out a cool oneliner as he does so. Dorn then beats Fulgrim so hard he pussies out of the whole siege, leaving the two to a bunch of his champions, including Eidolon who is stated to have the strength of a primarch. Not that it helped him any though since Sigismund basically just fed him his own sword before kicking him off the rampant in one of the single coolest fights of the book.
At the Eternity Wall Space Port, as she goes to meet her end, Krole remembers her achievements. She remembers the sacrifice of her Custodian companion Tsutomu. She is accidentally killed by Kharn who didn’t see her due to her null field. Meanwhile, Cambra Diaz's sword breaks, but he still fights on, until he finally dies defending a bridge, never taking a step back. Shiban Khan, who had grown close to an Imperial Army captain, tries to evacuate his comrades, as they are over run by World Eaters. Trying to use a shuttle to help them escape, he finds the pilot and his captain friend dead from bolter fire, before he himself is killed as the shuttle crashes.
With this, Saturnine, Gorgon Bar, and Collosi are safe, and though the Eternity Wall Port has been taken, it cost the forces of Chaos many of their very best. The entire Mournival, save for Abaddon, is destroyed, and 3 entire companies of the Sons of Horus, including the elite 1st Company have been destroyed to a man. And though Fulgrim may have escaped, doing so cost him 18'000 Emperors Children, not to mention all those champions that were just slaughtered by Dorn, Sigismund, and Thane, with very few Imperial Fists lost in exchange. Pert then changes his target from the Imperial Palace to the Imperial Fists (then still standing) Fortress-Monastery. Sanguinius then tells Dorn that Nuceria, the World Eater's home world has been destroyed; not just devastated like it was by the battle between Angron, Lorgar, and Guilliman during the Shadow Crusade, but proper Alderaan-ed by the Dark Angels. Speaking of the Dark Angels, at the very end, Corswain approaches the Phalanx with his half of the Legion, swearing to fight the traitors.
Duel of the Emperor and Horus
Like with any truly epic event, the siege only ended with the most motherfuckingest duel in the entire 40k fluff; the Emperor of Mankind against Horus, most favoured of the Primarchs and the living avatar of the Chaos Gods. If the Horus Heresy was the most important of a series of events, if the siege was the single most epic event, then the duel is the defining moment of the fluff and affected everything else that came after it.
During the duel, Horus had managed to mortally wound the Emperor (despite being a nigh-unkillable Perpetual himself), and would have finished him off, if not for the intervention of one Perpetual Guardsman(Oll Perrson/Ollanius Pius)/Imperial FistAstarte/Custode. He jumped in front of Horus as he was about to strike the final blow, and was killed. (duh.) The Emperor, seeing how far his most favored son had fallen, decided "Fuck this", and OBLITERATED HORUS'S SOUL!
Although the Emperor managed to win and kill Horus, he was so badly wounded in the end he needed to be on 24/7 life support just to survive. So really when you come down to it,it was a draw; Chaos had been stopped then but only at an unthinkable cost to the Imperium.
/tg/ Connection
What, besides the fact that it's the most important event in the 40k universe? Fine.
The Siege of Terra is also the theme for the Horus Heresy board game, in which you reenact the Siege itself. There. Happy?
Not really.
The fa/tg/uy's explanation of the Siege Of Terra (for Dummies and BL Editors)
The main rule of warfare. As the number of combatants increases, the resemblance to complete uncontrolled insanity approaches infinity. And then you have to take into account the terrain...
Basically you start with a planet that's been nuked, polluted, and generally lived-in for a few hundred thousand years too long. And everyone on it is fighting everyone else, constantly. Your basic unit of land is the Bunker, Vault 101 style. There isn't any natural plant life left, of course, so all the oxygen is made in vats with the food. ""Luckily"" this means you can build anywhere that isn't intensely radioactive, and hence fight over those areas. Get Mega-City-One, nuke it and rebuild it a few times, and then you start to understand.
Then the Emperor comes along, and manages against all odds to conquer the place. Suddenly everyone isn't killing and dying all the time, and a population boom happens. So Emps organised the largest set of public-works since the first colony ships. He rebuilds huge areas of the planet, and creates the Imperial Palace, the Astronomican, and a buttload more of cool shit besides. And what he gets is effectively one giant city, the second largest (after Commorragh) in the universe. "Huge" just doesn't do it justice as a description. Neither does "Labyrinthine", "Over Populated", or "Gothic Nightmare". And this New Terra was mostly just thrown over the original foundations of whatever was there, like a pile of gold-bricks onto a rat maze. There are bunkers and emplacements still around that date back to the War Against The Men of Iron and even before.
Then the Heresy came, and the Emperor says to Dorn "Fortify this fucking madhouse". So now everything that didn't have a gun emplacement before does now, everywhere. And he walled in half the doors and windows, put hundreds of AA batteries on every roof, filled entire rooms with concrete just for a bit of reinforcement, and generally panicked because if they would only ever be necessary if the space defenses (the best in the galaxy bar none) have failed.
Then Horus arrives in orbit. He's punched through the space-defenses at massive cost, but the war in space is far from won, and the Palace is just a flat no-fly zone, so he can't just pick and choose landing areas. So he bombards everything his ships can reach, fills the sky with Drop Pods, and tries to march on it.
Which is when Rule #1 kicks in and everything immediately gets Mega-Fucked, for both sides. Ruined streets and bunkers make navigating a nightmare, communications are somewhere between impossible and actively detrimental, Drop Pods land off target, plans and back-up plans fall apart in seconds, demons run amok. And the Primarchs are either constantly trying to out Tactical Genius each other or are too in the thick of it to relay any commands, so no-one has a fucking clue what's actually going on in the big picture. It's Stalingrad writ large, but without even the merest hint of sanity, and a thousand space marines charging into every breach. The inclusion of cackling daemons, rampaging renegade guardsman, and bellowing daemon engines doesn't help the situation.
So it takes roughly 10 minutes of this menial bullshit for a load of the Chaos forces to get bored and just decide "Fuck It, Let's Just Wreck The Place". So now everything makes even less sense, entire companies ignoring sensible objectives to go on the Chaos Marine equivalent of a bender. The Emperor's Children rape, murder, and pillage the civilians of Terra so hard that even 10,000 years later they still live in fear at the memory, while the World Eaters hack and slash at anything they think might bleed. Only the Iron Warriors and the Sons of Horus are whole-heartedly tearing at the palace, dedicated to rubbing it in Dorn's face like a bitch no matter what. And to the horror of the loyalists, they're succeeding. Brick by brick the greatest military stronghold in the galaxy is falling.
Which sounds great for Chaos, were it not for the simple fact it wasn't falling quickly enough. It was taking days to advance inches at massive cost, and Guilliman was en-route with reinforcements, with Russ and the Lion and the remains of their Legions right behind him. If the siege wasn't ended before they got there, the traitors would likely lose. So Horus put all his cards on the table and lowered his battle barge's shields, goading the Emperor (who didn't know about the reinforcements- or maybe he did and was enacting a much greater scheme, see below) on-board to hopefully kill him and force the defenders to rout. Everything else is history.
The short story "The Board is Set" seemed to indicate that the Emperor and Malcador knew about the reinforcing loyalists from at least the beginning of the siege and were fully aware of the siege's outcome up to and including the Emperor's ascension to the Golden Throne. It also seemed to imply that Horus lowering his shield may have been so he could teleport down and attack the Emperor, not realising it was Malcador on the throne. Possibly the most epic level of Just As Planned.