The Battle of Ragnarok was a definitive moment for the Revenants Chapter of Space Marines, one which has become among their most famous battles.
The Ragnarok system was the name given to a system of habitable worlds held by the Ork Warlord Grashkar Gitface, from which the warlord was known to launch attacks on Imperial Space. The authorities in the region grew tired of fighting of Ork raiders ad eventually a decision was reached to conquer the Ragnarok sector utterly, and so end the threat the Warlord posed. In early M41, the Imperium ordered the sending of a company of Revenants Space Marines and three regiments of Imperial Guard to do so.
At first, the Revenants and the Imperial Guard encountered only light resistance, however as they continued through the system, the resistance got heavier and heavier. Most of the planets were sparsely populated, and thus easily captured, but Ragnarok IV was held by the elite of Grashkar's armies, as well as being the home of Grashkar himself. Three months after arriving in the system, Captain Bane of the Revenants decided to take the planet; realising that if he cut off the head, the resistance posed by the body would be smaller.
Upon landing the Imperial forces were met with staunce and brutal resistance from the Orks, who were determined to hold the planet for themselves and drive the Imperial forces back. Captain Bane himself was killed three days after arriving, command of his company passing to Brother Sergeant Yan Rill, who decided to take his Captain's plan to its logical conclusion, and lead the Fourth company of the Revenants in an assault on Grashkar's palace itself. His plan - a tactic adapted from his youth as a trainee Marine - was simple. The remaining Imperial Guard would launch a diversionary assault on the capital settlement of the planet while Rill and the Revenants - together with a handful of elite Guardsmen - would attempt to storm the Ork palace with a lightning assault and kill Grashkar himself.
The palace was hardly deserving of the title, consisting of a half-kilometre tall, kilometre long pile of barely-organized junk turned into a building. Within a maze of scrap-metal corridors, the Space Marines fought, but the Guardsmen were quickly all slaughtered, unable to survive the brutal fighting. The Revenants however, excelled themselves and eventually cleansed the entire building. Yan Rill himself found Grashkar, a large, hulking Ork with a great power claw, and engaged him. Although outmatched in size, the Sergeant's faith and skill were such that he defeated the foul creature, and cast his body down from a balcony. With Grashkar's death, the remaining Orks had lost their figurehead, and were slaughtered by the victorious Imperial forces.
Mop up campaigns
The Imperial Guardsmen were left to mop up any surviving Orks and begin setting up Hab-Domes for the incoming colonists. They were supported in these tasks by elements of the Third and Fifth companies of the Revenants, who came in as relief for their brothers. These campaigns were made more difficult by the fact that many Ork Warbosses had kept their heads during the route and were still a threat; nonetheless, members of the Third Company of the Revenants managed to comport themselves well and destroy the Orks. It was during these mop up campaigns - brutal affairs, with the remaining Orks fighting tooth and nail for every inch of ground - that Aldus Flynn earned a promotion from Battle Brother to Sergeant.
The End of the War
When the last Ork Warboss was slain, it was almost five years after the battle of Ragnarok had begun on the outer planets. The planets now serve as Agri-Worlds for the Imperium, and little to no trace remains of the former Ork Empire.
The majority of the Revenants involved in the war retured to Peace, their home planet; amongst them Yan Rill, who was promoted to full captain of his company. Aldus Flynn was sent to other battle zones with his company as well.