Bloodborn
The Bloodborn was a chaos warband led by the biggest bastard in the 40k universe, the Iron Warriors Warsmith Honsou.
History
Honsou had been engaged in a little science project to make his old daddy chaos gods proud, which involved creating supermen (space Marines) by shoving preteen boys back into wombs of immensely obese women (the single most grimdark of grimdark moments in the fluff and which made Freud have a severe nosebleed). Thankfully Uriel Ventris turned up and wrecked that and Honsou's grand company to pieces. This made him just a little upset.
He decided the best way to show how ticked off he was by killing every ultrasmurf in the galaxy….and then smashing their worlds apart for good measure. You can’t say chaos doesn’t do things by half. To achieve this end he took part in the Skull Harvest, where chaos warbands and mercenaries would compete and whoever won would gain the right to lead the assembled host. After a number of bouts of bitchslapping Honsou gained that right and led his new delightful band of scum and villainy off to trash the smurfs. On the way they hooked up with M'Kar the Reborn, daemon prince of chaos who became the patron of the host. He named the warband the Bloodborn, for no reason other then trolling Honsou who clearly disliked the name.
(Just a background note, M'kar was a former Word Bearer who took part on the assault on Ultramar during the days of the Great Crusade and achieved nothing of note and was left to die by his fellows. For some reason though chaos granted him daemonhood. The qualifications for daemonhood weren't very strict back then.....)
Honsou's first act with his new force was to destroy the world of Tarsis Ultra which Ventris and his company had fought so hard against the Tyranids to defend against. This is proof that Honsou clearly is a few planets short of a system, as he was willing to waste men and resources and destroy an inhabitable world (a rare prize in the vast galaxy) just to spite someone.
He then began his assault of Ultramar and against the Ultramarines but here things started to fall apart. He was beaten after a series of intense battles and the Bloodborn was broken and shattered. M'kar was banished back to the warp and Honsou fled Imperial retribution, only pausing to shake his fist like a Saturday morning cartoon villain and scream he would be back. Giving though his ability to get shit down, this might be truer then we care to think….
Organisation
The Bloodborn was a highly mixed force, being centred on the remains of Honsou's great company and other allied Iron Warrior warbands. Other factions that took part in the Skull Harvest or joined up onroute includes some Kroot mercenaries; a Loxatl warband; corsairs; blade dancers; and several bands of traitor marines from the Claws of Lorek, the Skulltakers, the Apostles of Mithras, and the Death shadows.