Slaanesh is the Dark Prince, the God of Excess and Lord of Pleasure, and the most beautiful of all the Chaos Gods. Slaanesh's throne stands empty, however, for the god himself is lost. His minions wring their hands in anguish, while the remaining powers rejoice.
Slaanesh has been missing in the Mortal Realms for aeons, captured by the aelven gods between Ulgu and Hysh. Bound by magical energies, Slaanesh is trapped utterly, forced to relinquish those souls he consumed when the world-that-was fell.
Trapped somewhere between the Realms, Slaanesh has also had an insight into the dark future that could await the Mortal Realms, and is deliciously tormented by what the visions foretell – for one, it would make escape that much more difficult, and for another, the dominion of Death would just be unbearably dull.