Bringers of Despair
The Bringers of Despair are an elite fighting force within the Black Legion which serves as the personal bodyguard of Abaddon the Despoiler. Selected from the strongest and most vicious Chaos Terminators, these fearsome warriors are a precursor for the arrival of the Despoiler himself.
The big kids on the block of chaos dudes. Originally drawn from the surviving members of the Justaerin terminators, i.e. the original bodyguard of Horus--nowadays those survivors are supplemented by Abaddon handpicking the 'ardest motherfuckers from all the warbands and having them duke it out for a spot on his own personal bodyguard. In short they are the toughest warriors the Black Legion has to offer, with the strongest of Chaos' toys, acting as the bodyguard for the most dangerous of Chaos' champions. Not folks to mess with. <= This rule is to be ignored if your name is Thalastian Jorus or you are in the Death Company, in which case you can proceed to rape them.
The Talon of Horus gives them an interesting origin story. After a daemonic incursion when the warp core of their spaceship detonated, the remnants of the Justaerin somehow managed to simultaneously become possessed by the same Daemon. Although its implied the Daemon spread itself too thinly to cause the standard gribbly mutations the stereotypical Possessed have, the Daemon is able to remain in the material realm as long as any single one of their hosts were alive, and the Justaerin became bound together in such a way that granted them an unnatural unity and co-ordination. It is unknown whether Bringers of Despair who were not part of the thirty original possessed Justaerin have also become possessed by the Daemon.
On the Tabletop
The Bringers of Despair used to be an upgrade for a single Terminator unit where each member could buy BS and WS 5 for 3 points apiece. Combined with the Black Legion Veterans of the Long War tax and any marks you want on them, this made for quite an expensive unit. Nowadays, you can just field the Bringers of Despair formation, which consists of Abby and a single unit of Terminators. The Terminators get WS and BS 5, and can re-roll a single "Look out, sir!" roll for Abaddon each turn.