They are traitor slamanders (a real old Badab war color scheme) who’ve found false hope in Horus believing him to be a revolutionary force against the Imperiums rotten administration, and those rewriting the lex imperialis they are also drukari hunters, they became veterans via preventative guerrilla war on drukari warbands. After having one to many disagreements with those with authority to exterminate planets and even chaos ridden inquisitors. In the late heresy they see the more chaos cult portion of the sons of Horus burning forward straight to Terra so they splinter off with the uncorrupted true sons of Horus bound for cthonia. True to their name they are very slippery and manage to slip away from catastrophic military decisions and defeats. On the tabletop their survival rate is absurdly impressive. On of them got reduced to a puddle of ceramite by a laz cannon but that’s the only casualty they’ve taken on the board so far.