I wrote all my stuff from the ground up, I was never a fan of rolling for random fluff.
My marines were founded in M33 or 32 as Imperial Fist successors and were in charge of protecting a colony system which was going to host spaceship foundries, but when they moved to the system to colonize they found a Knight House which they buddied up with after some confusion.
In M35 they met the Titan Legion who they became big friends with, and in M40-M41 the marine's home system was attacked by orks twice in ten years and both times the Eldar prevented their cries for aid from being heard. The Space Marines lost most of their chapter, the Knight house went from about 100 knights to 14, and out of 2 billion citizens only about 70 million remained.
Later the Dark Eldar came to learn this and, after a change of power in the Kabal, the new Archon, who hated that craftworld because she was once a scourge but the head Autarch ripped off her wings, leaked it to the Space Marines who got all their buddies together and butchered the Craftworld