Slaanesh is the God of Decadence and Depravity. His chief rival is Khorne and his sacred number is six.
"Take care, lest your protests grow tiresome. I have asked for so little! Anyone would think I had asked you to sacrifice yourselves and all your family. And yet, in Slaanesh's boundless and pleasing mercy, I have asked only for your daughters. Surely you would not deny me my small enjoyments?"
- Tyrell, Renegade Lord of Arden IX
Slaanesh is the youngest of the Chaos Gods and is known as the Prince of Chaos. He came into being when the Eldar fell from grace, created by the decadence that had corrupted the galazy-spanning Eldar empire, and which his birth destroyed. In the moment of Slaanesh's birth the Eldar became a doomed people, reduced to a tiny remnant of refugees scattered throughout space. Worse still, they knew that if not for them, the Chaos God Slaanesh, the Depraved One, the Doom of the Eldar Incarnate, would never have been born.
Alone of all the Chaos Gods, Slaanesh is divinely beautiful. He is seductive as only an immortal can be, disarming in his innocence, utterly beguiling in his manner. He is a master of luxury and over-indulgence, of cruel and unnatural passions, of hidden and terribly corrupting vices, and of all the cruel temptations that only a god can offer.
Slaanesh can assume male, female, or hermaphrodite form at will, and it is impossible for a mortal to look upon his divine face without losing his soul and becoming a slave to his slightest whim. His followers revel in perversity and debasement and take delight in all manner of abominable and unclean deeds. They have abandoned the last vestiges of true decency and sacrificed themselves to serve the Pleasure Lord in the endless pursuit of new experiences, no matter how vile or depraved these may be.
Taken from Codex: Chaos v2
Lose your soul to the immortal beauty of Slaanesh.