Background
One of the Renegade Chapters, the Warp Ghosts, betrayed the Imperium and turned to Chaos, the Chapter allying themselves to Abaddon's 13th Black Crusade2. Being only a chapter, they have far less marines and equipment compared to the Chaos Legions, but are renowned for their ferocity.
Their chapter colour is a dark green/white with black and gold details; the renegade chapter's symbol is yet to be seen.
The Bleak Brotherhood is believed to be a fraction of the Warp Ghosts.3
Notes
Based on the colour scheme, the Warp Ghosts may be another name for the Death Mongers, another Chaos Space Marine warband. This may simply be an oversight by Games Workshop.