From Lexicanum

History

Little is known about origins of the Imperator Titan Aquila Ignis, as it was first revealed by the Heretical Legio Mortis at the beginning of the Schism of Mars.

The titan was deployed by Legio Mortis during their walk on the Magma City and the defending loyalist Legio Tempestus. Though Aquila Ignis succeeded in destroying the remnants of Tempestus, the destruction of the Magma City by Adept Koriel Zeth unleashes a torrent of lava that destroys the Imperator's main body, leaving only its Hellstorm Cannon behind.

Involvement in Dawn Of War: Dark Crusade

The remains of the Titan's massive Hellstorm cannon were found in Ironworks Bay by the 1st Kronus Regiment. Techpriests had brought this relic to Kronus millennia ago, after it was cut from the heretical Titan during the Horus Heresy. The Cannon was restored to half-functional status and served in the defense of the Guard's planetary capital.

If enemy forces successfully overrun Victory Bay, Captain Gregor Vash orders a Techpriest to "overload the core" to prevent the fall of the Titan cannon to the attackers. The Titan cannon begins to overload and explode, the blasts taking out the nearby combatants. However, if Chaos were the victors, it is implied that pieces of the Titan cannon still remain, as Eliphas' sorcerers released a daemon of rot and plague (which means it belongs to the Chaos god Nurgle) from the cannon through a ritual of sacrificing the corpses from the attack. The daemon had likely resided in the cannon for ten millennia since the traitor Titan Aquila Ignis last walked during the Horus Heresy, and by releasing it, Eliphas gained Nurgle's favour.

If the Imperial Guard emerges victorious in the Dark Crusade, however, vast numbers of Techpriests arrive on Kronus with much-needed supplies for the Guard in gratitude to General Lukas Alexander's recovery of the great Titan Gun. They then attempt to fully restore the Hellstorm Cannon to the Emperor's service, never knowing of the daemon that lay within.

Sources

• Mechanicum by Graham McNeill
• Dawn of War: Dark Crusade