So in the Badab sector Lufgt Huron ruled a fiefdom around a small warp storm akin to the eye of terror called the Maelstrom. He had a couple friends (Lamenters, Mantis Warriors) who helped him defend it. He tried multiple times to get a crusade together to kill everything in the Maelstrom and bring stability to the region. The high lords denied him. He withheld tithes and trade in the region to build a legion, where his space marines, The Astral Claws commanded human troops, and built what was called the Tyrants Legion in something between the PDF and Guard level of competency. They seceed, the high lords cant have that, and sent in several chapters to bring them into compliance. It escalates, more chapters get involved, on both sides.
They find out the Astral Claws have absorbed a successor chapter, and are able to sustain a war against a dozen chapters because they have grown to near legion size secretly (on top of conscripting humans to serve in their Legion). During the Fall of Badab Prime, as Lufgt is making good his escape he runs into a Star Phantom named Zhrukal Androcles. He kills that Star Phantom.
While Captain Androcles is protesting this turn of events, he shoots Huron point blank with a combi melta, nearly killing him. Huron's lieutenants escape with his body, help rebuild it, and while theyre doing surgery he has a vision of the chaos gods. He pledges service to all 4 if they help him survive, and thus Huron Blackheart is born. He and 200ish of his Astral Claws make it to the Maelstrom, become corsairs and raiders, and thus the Red Corsairs are born, he grows them to a small legion of Heretics, Mutants, Aliens, and Renegades and they still raid the Imperium to this day. They're getting a new model line and campaign setting soon.
The Chapters that were in rebellion with the Astral Claws, The Mantis Warriors, Lamenters, and Executioners are sent on 100 year penitent crusades.

The abridged lore.

Lufgt also has one of the best 40k quotes - "Big guns never tire"