Well, a lot of the mystery surrounding them is gone as they know where the Legion comes from and what happened to make them into what they are. Used to be the Fire Hawks Chapter that got lost in the warp, picked up some kinda warp plague that eventually makes you go insane as you deteriorate, but not before it makes you ungodly powerful. Lost over 800 Brothers to the warp accident so only 200ish left after, and dwindling. If you're going to die, might as well take as many of the bastards with you! They really shouldn't be in game unless BI has some kind of super massive event and it's a one time thing they show up for a little fluff or something...but even then it's not really needed.