Bleak Brotherhood
The Bleak Brotherhood is one of those obscure as fuck warbands that nobody has heard of unless they are reading about other warbands on the Fandom Wiki or Lexicanum. Anyway, these guys are supposedly a Warp Ghosts Splinter Warband which have a bit of lore about them.
During the Magdelon Incursion in 712.M41, this formation of Chaos Marines, makes a contract [is it called a contract, I have no fucking idea] with some daemonic power to aid them in the upcoming wars, but the price was to destroy the crystal city of Magdelon. In doing so, they caught the attention of the Space Wolves, and by golly this pleased the Daemonic Power because the city got ruined and the incursion started. The Brotherhood lost though and fled to the Lycanthos Secundus.
Now we're getting to their other small part in lore, in the Lycanthos Drift Campaign where they took over the Bastion known as the Widowmaker. This Stronghold withstood numerous Imperial attacks for three decades up until Lugft Huron and the Astral Claws came around and fucked those Bleak bastards up.
Last time they were seen was during the 13th Black Crusade, assaulting the Cadian Gate with plenty more Warbands. If they escaped it's anyone's guess at this point. Weirdly enough, Bleak Brotherhood have the same color scheme and symbol as another warband known as the Apocalypse Company. If its laziness from GW or Apocalypse Company is another group splintered from the Warp Ghosts, we will never know.
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We don't know why we exist