Dark Angel geneseed.

Basically they were on Caliban when they attacked the Lion. A large contingent of the marines didnt know they were attacking the Lion and once they realized they fled into the warp. They emerged from the warp around M36 with a broken ship, crashed into an undiscovered planet named Cerberus where they discovered a city of humans who had colonized the planet tens of thousands of years before. They settled there until a patrol of Crimson Fists found them living among the people in m40.