Editing Siege of Terra (section)

==Assault on the Mercury Wall and Recapture of the Astronomican==

With the repulse at Saturnine and the fall of the Eternity Wall spaceport, the Siege was entering a new phase. With both of the Imperial Palace's primary spaceports in their hands, the traitor forces began bringing in all their reserves and materiel stores, preparing to overwhelm the loyalists through sheer numbers. Perturabo was still directing the battle more or less singlehandedly at this time until he received a summons from Horus to attend him on the ''Vengeful Spirit''. When the Lord of Iron arrived in Horus' throne room, the Warmaster instructed him to abandon his current battle plan. Instead, he wanted to throw everything they had, including the Titan Legio Mortis, straight at the Mercury Wall, which represented the true beginning of the Imperial Palace. Perturabo demanded to know why Horus wanted to employ such a wasteful and apparently futile strategy, and Horus stated that it would work because he willed it so. Shortly thereafter, Horus sent his equerry to Perturabo with orders to disperse the Iron Warriors among the traitor forces. He followed up this humiliating order by informing Perturabo that Mortarion and the Death Guard would be taking over the IV Legion's positions. Infuriated, Perturabo denounced Horus' alliance with the Ruinous Powers and declared that this was no longer a war of Legions, but a war of foul and unnatural powers in which no true victory could be won. He then bitterly declared that Horus was exactly like the Emperor: both of them had manipulated Perturabo from the very beginning and forced him into a role he despised, that of the ruthless, calculating siege master. With that, he ordered the IV Legion to withdraw from the battlespace. A few diehards chose to remain behind, but nearly the entire legion obeyed their primarch's order. Some of the traitor forces attempted to stop the Iron Warriors as they headed for the exits, but were unsuccessful.

Unbothered, Horus ordered the attack on the Mercury Wall to proceed, spearheaded by Legio Mortis. To counter the Death's Head, the loyalists deployed the Legio Ignatum and a few Titans from Legio Solaria, along with Knight banners from Houses Vyronii, Tyranus, Cadmus, and Konor. A representative of the Mechanicus attempted to convince Ignatum's Titan drivers to flee the battle, as his calculations had shown that defeat was inevitable, but the principes rejected his proposal and walked to war anyway. Also present were a number of other Titans from legions that had been decimated at Beta-Garmon, but many of them refused to join the battle, citing the Titandeath as their reason for remaining out of the fight. This lasted until the engagement between Mortis and Ignatum began, in a vast open space known as the Mercury-Exultant killzone. The traitors were revealed to be using Titans that had been destroyed at Beta-Garmon and elsewhere as cannon fodder; the wrecked Titans had been reanimated via sorcery and now teemed with blight and corruption. Ignatum smashed through these revenants, only to be confronted with the main strength of Legio Mortis. A desperate battle ensued, with dozens of god-engines being destroyed on both sides. Proscribed weapons such as warp and vortex missiles were employed freely, for this was now a battle of annihilation. Recognizing the import of the engagement, the Emperor communicated with a representative of the Ordo Sinister, the commanders of the dreaded Psi-Titans, and ordered him to join the battle. One of their prefects made himself known to Dorn, who agreed to deploy the four available Psi-Titans into the battle. He then took command personally at the Mercury Wall, bringing reinforcements with him. Ambassador Vethorel of the Adeptus Mechanicus approached the Titan crews who had refused to join the battle and showed them images of the reanimated Titans being used by the traitors. Galvanized by the desecration of their fellow god-engines, the Titan crews agreed to rejoin the fight. Vethorel proclaimed them to be a new Legio, the Legio Invigilata. Led by the former Grand Master of Legio Solaria, Invigilata joined Ignatum and the Psi-Titans on the front line. In spite of the loyalists' bravery, the main strength of Ignatum was destroyed by the superior numbers and firepower of Legio Mortis, combined with an orbital bombardment from the traitor fleet. The survivors attempted to rally and continue the fight, but Mortis had reached the Mercury Wall and began to tear it down.

During this battle, Corswain of the Dark Angels was conferring with Admiral Su-Kassen and other leaders of the Imperial fleet. Corswain had been expecting to find the rest of the I Legion already present at Terra and was dismayed to learn that he and the forces under his command were the only Dark Angels in the system. He had brought only ten thousand Astartes and two dozen ships with him, barely enough to make any kind of impact against the enemy forces in orbit. Unwilling to sit by and do nothing, Corswain announced that he intended to recapture the Astronomican, which had fallen into traitor hands and gone dark. Without it, the I and XIII Legions would be unable to reach the system and relieve Terra. Some of the Dark Angels in his fleet, having been subverted by Luther's separatist faction, wanted to assassinate Corswain to avoid being wasted on what they considered a pointless suicide mission. They were talked down by Librarian Vassago, who was a member of their faction but admired Corswain's bravery and nobility. Admiral Su-Kassen agreed to lend them the ''Imperator Somnium'', an immense battle carrier that had served as one of the Emperor's personal flagships, for their attack. The Dark Angels proceeded to use the ''Somnium'' as a sort of fireship. Concealing their own vessels under its tremendous bulk, they rode in with the huge flagship as it drew the fire of the entire traitor fleet, then split away and charged through to the Astronomican before the traitors realized what was happening. The ''Somnium'' died hard, taking many enemy ships with it and inflicting critical damage on the ''Conqueror'' and ''Terminus Est''. The Dark Angels successfully landed at the Astronomican and breached its defenses. They found that the mountain had been overrun by elements of the Emperor's Children and Vassukella, a Daemon Prince of Slaanesh. Despite sustaining heavy casualties, they were able to kill Vassukella and the corrupted Children, reclaiming the Astronomican for the Imperium. Corswain was nearly killed by the psychic backlash of the daemon's death, only to be saved by Vassago. The news that the Astronomican was once again in friendly hands provided a much-needed morale boost to the Imperial forces, though this was offset by the grim news from the Mercury Wall.

Subsidiary combat continued all along the Palace's defensive perimeter, though it was comparatively small in scale when measured against the annihilating fight taking place at the Mercury Wall. The loyalists were beginning to reach the limits of their mental, physical, and spiritual endurance, though some of them took confidence in a new credo: "He protects us as we protect Him." Even so, the loyalists' morale was being further eroded by the malign influence of the Warp. Those who were sensitive to its currents and eddies noted that its strength was waxing as the Siege ground on, working its way through the cracks in the Emperor's wards and battering down the mental defenses of the loyalist troops. Suicides, murders, and desertions spiked as exhausted and despairing soldiers and civilians sought to escape into a paradisaical dreamland. Unfortunately for them, this dreamland was a trap laid by the Emperor's Children to prey on the desperate and fearful. Thousands of unfortunate souls were lured to the Hatay-Antakya Hive, where the III Legion entrapped them in their dreams and "milked" them for their emotions. These activities were disrupted by the arrival of [[Ollanius Pius|Ollanius Persson]] and his band of refugees from Calth, who were seeking to rendezvous with John Grammaticus and his prototype Space Marine bodyguard Leetu. They in turn were aided in their escape by a mysterious woman calling herself "Actaea" and a legionary in scaled armor who identified himself as Alpharius. Together, this unlikely group of allies embarked on an unspecified mission involving the Emperor.

Back at the Ultimate Wall, loyalists were succumbing to the despair field and insanity of the Warp, including at pivotal gun sections. Despite sane loyalists attempting to stop it, a group of maddened tech-priests sabotaged the shutdown mechanism of a section of wall guns, causing the guns to continue firing and risking the reactors powering them. The guns subsequently failed and exposed the reactor to incoming fire from the Traitor Titans. Despite loyalist efforts to avert disaster, the reactors were destroyed by incoming fire, resulting in chain reaction that caused a kilometers wide breach in the Ultimate Wall. While the rubble field delayed the Titans for a time, it would not stop Traitor infantry and Armour from scaling the debris.

The Traitors were now inside the Inner Palace.