The ringed planet therefore played host to a battlegroup consisting of two dominator class vessels, Black Prince and Warrior, supported by the carriers Illustrious and Intrepid. These ships remained on station, but were at risk of being caught unawares, the warp rift hindering sensor and psychic sweeps, potentially allowing a surprise attack by enemy forces. This is precisely what happened.
Admiral Thok managed to slip four cruisers, including the heavy Styx class Hand Of Loki, into the system without the Imperial battlegroup detecting them. When they began their attack run all but Black Prince quickly reacted to the attack, but as the Chaos fleet split up the Warrior found itself in a one-sided firefight with the Hand of Loki and another cruiser, while the other two Chaos ships were attacked by the carriers Illustrious and Intrepid. Warrior, crippled, soon disengaged, and the main Chaos force continued their attack, catching Black Prince on the far side of the planet Simius.
Four Iconoclast cruisers also launched a suicidal attack on the Intrepid, throwing caution to the wind and using the warp rift to jump into the space between the planet Simius and its rings. There, after causing minor damage to Intrepid the escort vessels were blown to pieces by Black Prince and the attendant system ships. Intrepid then moved off to engage two Chaos ships, along with her larger carrier consort Illustrious.
The Chaos cruisers Nebelung and Wyrmfire, the latter having been crippled early on from a lucky nova cannon shot, found themselves outgunned and outfought by the Imperial carriers. Soon the Nebelung exploded allowing Wyrmfire to drift away. Now the Imperial carriers tried desperately to aid their comrades aboard Black Prince who were coming under a sustained and brutal attack from the main Chaos battle force.
Fortunately for the Imperium the attack craft launched from Hand of Loki were clearly inexperienced, causing little damage. In the one-sided engagement the Black Prince was eventually reduced to a hulk, but not before the Syx class Chaos ship exploded in dramatic fashion.
Brunhilde, the last remaining Chaos vessel then made a last ditch charge against the Imperial carriers, quite to the bafflement of Captain McManners on Illustrious. The attack failed, Brunhilde being reduced to a wreck and then boarded by Imperial stormtroopers. Most of the crew were dead and the prize was taken in tow. Wyrmfire was never found.
The attack was a setback for Admiral Thok, who flew into a rage when he heard that three more of his cruisers had been destroyed or captured and a fourth was missing. The Imperium had lost Black Prince temporarily, but she would be refitted, probably along with Brunhilde at Caitlen station. In the long run the battle would be a net gain for Admiral Jellicoe.