The Dies Irae ("Day of Wrath" in High Gothic) was a massive Imperator-class Battle Titan assigned to the Death's Head Legion, the Legio Mortis, of the Adeptus Mechanicus' Collegia Titanica which served during the final days of the Great Crusade as part of the Titan contingent assigned to the 63rd Expeditionary Fleet of the Warmaster Horus and his Sons of Horus Space Marine Legion. The Dies Irae was commanded by Princeps Esau Turnet and his two Moderati Primus were Titus Cassar and Jonah Aruken. When the Horus Heresy began during Horus' massacre of the Traitor Legions' remaining Loyalists at Istvaan III, Princeps Turnet and Jonah Aruken chose to side with the Warmaster while Moderati Cassar, a devotee of the early Lectio Divinitatus cult which believed in the divinity of the Emperor of Mankind, opposed Horus' betrayal. Aruken killed Cassar and the Dies Irae ultimately took no action to stop the Massacre of Istvaan III when the Traitors' virus-bombs began to fall upon the Loyalist Space Marines of the Sons of Horus, World Eaters, Death Guard and Emperor's Children Legions. Ultimately, the Dies Irae joined the forces of the Dark Mechanicus and it and its crew were twisted and corrupted by their exposure to and willing service of Chaos. The Dies Irae met its end at the hand of Imperial Titans in 999.M41 during the 13th Black Crusade on the Forge World of Hydra Cordatus.
When the Warmaster Horus ordered the 63rd Expeditionary Fleet to unleash a virus-bomb attack on the one-third of the Space Marine Traitor Legions' forces that were fighting against the Slaaneshi-corrupted rebels on the surface of Istvaan III but had remained loyal to the Emperor, Princeps Esau Turnet had already chosen to support the Warmaster. He ordered the massive Titan to stop its support of the Loyalist Death Guard Space Marines it had been deployed to aid in the assault and then initiated the great machine's protective protocols against nuclear, biological and chemical attack, sealing it airtight against the deadly virus. The crew watched in horror as the virus-bombs' lethal cargo killed all organic life on the planet within minutes and withstood the hideous firestorm that engulfed the planet when Horus' fleet ignited the flammable gases of decay now present in the Istvaanian atmosphere. The Titan's crew barely weathered the firestorm as the Dies Irae's internal cooling mechanisms were nearly overwhelmed. One of the Titan's Moderati Primus, Titus Cassar, a devout believer in the divinity of the Emperor, turned against Princeps Turnet and tried to kill him with his sidearm, but was murdered in turn by his own friend and fellow Moderati Jonah Aruken, whose ambition to one day become a Princeps himself and take control of a powerful Battle Titan overwhelmed his moral sense. Believing that if he stood against Princeps Turnet he would never receive the command he so desperately hungered for, Aruken shot Cassar in the back with his own autopistol and fully committed himself to the Heresy. The Dies Irae was then deployed by the Traitor forces against their Loyalist former Battle-Brothers and the Titan was ultimately partly responsible for the death of the Loyalists' leader at Istvaan III, Captain Garviel Loken of the Sons of Horus. Loken had become trapped in the wreckage produced by the Titan following his duel with First Captain Ezekyle Abaddon of the Sons of Horus.
During the Siege of the Imperial Palace on Terra at the climax of the Horus Heresy, the Dies Irae participated along with the rest of the Legio Mortis and the other Traitor Titans in the operation to breach the palace's fortifications. The Dies Irae was the first Traitor Titan to breach the Imperial Palace's walls, which allowed the Chaos Space Marines and their allied legions of daemons to overwhelm the Loyalist defenders who rushed to fill the breach. This action earned the Dies Irae both infamy and the hatred of the Loyalist Titan Legions, who determined in the future that it would be worth the loss of an entire Legion of Battle Titans to destroy the greatest of the Traitor Titans. The Dies Irae survived the defeat of Horus at Terra by the Emperor and retreated into the Eye of Terror with the rest of the Traitor Legions and the Dark Mechanicus' forces, where thousands of years of exposure to the influence of Chaos slowly mutated the form of the once proud Titan and imbued its artificial intelligence with a fiendish, daemonic presence and aspect. It is unknown if the Titan remained crewed by Princeps Turnet and Moderati Aruken throughout its entire existence, but this is not beyond the realm of possibility, though like all the devotees of Chaos it is likely the two would have been hideously warped and mutated by their long exposure to the Immaterium.
The Dies Irae fought its final battle against the Imperium of Man as part of an Iron Warriors Traitor Legion warband that attacked the fortress of Tor Christo on the Adeptus Mechanicus Forge World of Hydra Cordatus in 999.M41 as part of the 13th Black Crusade. Now a twisted Dark Titan of Chaos, the Dies Irae had become the lead Battle Titan of its own Dark Mechanicus Titan battlegroup. It supported the Iron Warriors' assaults on the fortress and ultimately was responsible for the destruction of several Imperial Titans and the death of the Imperial Titan battlegroups' commanding Princeps. The Dies Irae met its end after a Warlord-class Battle Titan badly damaged it by intentionally detonating its plasma reactor in proximity to the Dark Titan, allowing another Warlord-class Titan to penetrate its damaged armour and destroy its main fusion plasma reactor with its Power Claw. The reactor went critical and incinerated the Dies Irae and its corrupted crew, finally earning the Collegia Titanica its vengeance after 10,000 years.
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