Isn't there a chapter that was lost during the Horus Heresy? I seem to recall that they appear at seemingly random times, slaughter an army, then vanish. Officially the Imperium of Man does not know where their alliance lies. I can't remember their name though.
I think you probably mean these guys: http://wh40k.lexicanum.com/wiki/Legion_of_the_Damned#.UqgOGLCYYdU
yea but i heard the legion of the damned (read the lexicanum, not a real legion) got some super lame backstory reveals with the Halloween ebooks Black Library released. The lexicanum isn't up to date with those ebooks yet. Probably better. Reply to OP: i think the exorcists are overrated and sound like matt wards work-too op and just overkill "yo i heard you like strong humans so i made the strongest humans ever called astartes and then made these stronger astartes who got a kid chapter and those astartes are stronger against daemons than other astartes and can kill daemons easily when other astartes arent strong enough." The Mantis Warriors and Carcharodons(Space Sharks) are pretty awesome though. MW for their"Battle-haze" mostly, their actions and aftermath of Babad war are just extra goodies. The fact that my favorite two lesser chapters also have rough history with each other just makes it even more awesome! http://wh40k.lexicanum.com/wiki/Mantis_Warriors#.Uqguj-LkfRc http://wh40k.lexicanum.com/wiki/Carcharodons#.Uqgu_-LkfRc
Novamarines due to color, emblem and their behavior. Always coming into action, nobody where needed! They are like the protectors of mankind! They are kinda like the Avengers that way! ^^
From this 5 option Exorcists they are cool chapter,emblem is interesting, Warcry is unique "Reports describe the Chapter using a slow, chilling chant in a number of dark languages, mixed with litanies in High Gothic" and also maybe Death Spectres. But there are some other cool chapters like Red Scorpions, Iron Snakes, Scythe of The Emperor but they are almost destroy.
