legion of the damned theories

New to the lore anf have been obsessively going through wikis and this page to learn all i can, came across the Legion of the Damned, astartes in jet black armor with basically hot rod flames (or ghost flames) decals that shoot incendiary ammo and basically blip into battles just when all seems lost only to disappear once the fightings over. I’ve seen theories about a legion that got lost in the warp (Phoenix’s?) and now before they get corrupted by chaos they’re using their power to strike at the enemy. Was wondering if anyone had any other info about them or theories about who they are and what they might be doing