Ka, this is all the fluff ive done so far for my chaos marine and ork armies.

Dread Mongers (The Dreaded Ones, The Corrupt)
Way, way back before the Horus Heresy, there was a chaplain of the Word Bearers who, apart from seeming completely normal to his brother marines, frequently had nightmarish visions while he slept. These visions spoke of a dark entity that lay within the Eye of Terror, and it called to him. With the events of the Heresy over, and the traitors scattered, the now twisted chaplain and his accursed retinue of four left the Word Bearers and explored deep into the Eye, searching for whatever called to the chaplain. If he found it or not, only he and those that joined him know for sure.
Recently, a small force has been reported attacking randomly in the Daemolcules Gulf, indescriminatly raiding and looting both Imperial and Tau settlements. Though small, the engagements have been growing in size. Survivors report a massive daemonic figure in Word Bearer colours leading the forces.
This creature is Vaevictis, the chaplain who ventured into the Eye so long ago, now a daemon prince. He and his retinue have returned, mutated and gifted by the Dark Gods. How and why they have appeared so far from the Eye is unknown. Vaevictis seems to be ammassing his own legion of mutants, traitors and renegade space marines, and it grows in power with each victory. There purpose is uncertain, but whatever it is, it can't be good...

Da Harlykwinz
The origins of the Ork Warband, Da Harlykwinz, is much simpler than that of the Dread Mongers. Warboss Zagsnikk Dakkadakka Grimgrub (so named for his habit of setting ambushes, forgetting he's waiting in ambush and then running at the enemy, killing a surprising amount with his trusty big shoota) saw some Eldar Harliquns slaughtering a mob of fellow orkses, and thought to himself "Day must be dat good cos of all the colourz dey're wearin." his next command to his boyz was that "All yer suits and buggies must be real bright and stripey!"
Zagsnikk's warband is now a brightly coloured carnival. Of orks in white facepaint. Thier luck or skill dont seem to have changed, but Zagsnikk is having too much fun throwing explosive pies at people to care