Hives and Hereteks

Black Crusade game.
Tone is Serious but not Grimdark.
Antiheroism and antivillainy is expected; on the Rogue end of the Rogue-Despoiler of Worlds spectrum.
There is no contractual obligation to fall to the ruinous powers, but that doesn't mean it won't happen.
Chaos Astartes characters are acceptable, though conceptually a (former) member of the Storm Wardens chapter who has recently gone rogue may be equally or even more appropriate. For balance purposes, any Astartes is either a relative novice or suffering some sort of amnesia - specifics TBD for any Astartes.