Hey, if you haven't got a history for your kabal yet you can have this.
Even after the fall, some Eldar still turned to darkness, unable to control their own base instincts. This normally happens to those who are outcasts. They flee to Commoragh in the webway, or are hunted down by their fairer brethren.
And to the idea...
Upon the _____ Craftworld (insert name there) Many trod the path of the outcast. Like Ulthwe, they remained near to the eye of terror. To near. The Seers quickly realised that there was something wrong with the younger generations. Too many were becoming outcasts, till there was more than 10 outcasts to every true craftworlders. And too many seemed to be falling into darkness. The seers realised it quickly, but not quick enough. Slaanesh had been able to touch the minds of these Eldar, twisting them. The craftworld fled its plight, to the distant reaches of the galaxy.
Outcasts began returning to the craftworlds. The residents thought that their curse had been lifted. Then, the seer, somehow blinded by bliss ('kay that idea needs some work..!) threw a feast, to celebrate. The venerable older members of the craftworld arrived first, seeing that the younger were late, thinking they were putting extra effort into their appearance for the occasion.
Then they arrived. jGuns firing wildly in their hands, knives stained dark with blood they slaughtered their Elders. The craftworld still exists. Some other craftworlds attempted to free it of their curse, but no attempt has succeeded yet. The Fallen craftworld may be the home of fallen, but they maintained its systems- And use it to attack. They could appear anywhere from the webway before: but places without portals could be assumed safe. Not any more.
That's it's rough form anyway. Anyone like it?