Battle of the Diavanos System
As the Horus Heresy neared its end, the World Eaters Centurion Deranax was given command of a number of his Legion's ships and charged with preventing the Blood Angels from reaching Terra. Though the Ruinstorm was weakening by that time, few paths through it led to Terra and, knowing this, Deranax prepared an ambush in the Diavanos System for the Sons of Sanguinius. However, Diavanos was a world of the Imperium and they sent out a plea for aid, once they discovered the World Eaters' presence. Miraculously, it pierced the Ruinstorm and was received by the Primarch Roboute Guilliman, whose Ultramarines were fighting any Traitors they came across to ensure the Blood Angels reached Terra. When he learned of Deranax's fleet, Guilliman ordered the Ultramarines to the Diavanos System, both to clear the way for Sanguinius, but also to save Diavanos. He viewed the world as an ideal example of what the Imperium strove to be and in saving the world, he hoped to ease his guilt in creating the Imperium Secundus.
Under their Primarch's command, the Ultramarines reached the Diavanos System before the Blood Angels, and caught Deranax's forces by surprise. A number of the traitor's ships fell in the Ultramarines' opening salvo and, despite the ferocity of their following attacks, all knew the World Eaters had lost the battle. However, even as his fleet was being destroyed, Deranax sought to land one last vengeful blow and ordered his stricken flagship, Gladiator, towards Diavanos.
Fearing that Deranax sought to perform an Exterminatus on Diavanos, Guilliman quickly invaded the Gladiator alongside a squadron of the Invictus and a Destroyer Squadron led by Captain Hierax. When they board the traitor warship, though, the World Eaters set off a series of explosions that blocked off any path to the Gladiator's bridge. As the Gladiator entered Diavanos's orbit, there was one option left to stop Deranax; Guilliman ordered the Destroyer Marines to cut a path to the Gladiator's torpedo bays. This played into the Centenarian's hands, however, as he had anticipated Guilliman would personally try to stop him and so had armed a Cyclonic torpedo to kill a Primarch and not a world.
When the Ultramarines saw what the Centenarian intended, they charged the World Eaters protecting the torpedo and Guilliman was able to kill Deranax before he detonated the explosive. The Ultramarines then disarmed the cyclonic torpedo, but were forced to escape from the doomed warship, as the Gladiator began to plunge into Diavanos. When it hit, the Gladiator devastated the world's surface. Although, thanks to Guilliman's actions, Diavanos still lived, it was left a mere shadow of what it once was. Guilliman deeply regretted not being able to preserve Diavanos, vowing he would continue to fight to ensure the Imperium as a whole did not share its fate.