Slannesh is young compared to the other Chaos Gods because his existence was due to a particular event called the Fall of the Eldar at the end of the Age of Strife. His/Her birth was caused through the unrestricted hedonistic activities of the psychically-aware, technologically advanced, culturally decadent and far-reaching Eldar Empire.
In the early history of the galaxy, the powers of the Warp had yet to form into distinct entities. At this time, the emotions of mortals flowed and ebbed as water does in a stream. As the mortal races grew and prospered, so did the strength of their emotions. Eventually, the gods grew to such a point where they could act independently of the general flow of emotions and thus became the Gods of Chaos. They reached into the dreams of mortals and demanded praise and servitude in order to increase their own power, as the of more one emotion is exhibited (in both thought and action) the stronger that god becomes.
The four main Gods of Chaos are:
- Khorne - The Blood God
- Tzeentch - The Changer of the Ways
- Nurgle - The Great Lord of Decay
- Slaanesh - The Prince of Excess
The first three gods became sentient by the end of M2 (Second Millennium, Earth-time) but Slaanesh didn't fully awaken until the Fall of the Eldar in M29 (29th Millennium, Earth-time).
The Eldar, an ancient race, when most of the mortal races of the galaxy were young, are known for their works of unfettered beauty and acts of unparalleled cruelty. For the Eldar, with their very long lifespans, their awareness of the nature of their psychic powers on the underside of the Universe, were very careful to keep their psychic powers under very strict control. Unfortunately, it was this very control that allowed the dark aspects of their powers to take root as Slannesh.
The Eldar understood their psychic effects on the Warp were dangerous and regulated the activities of their populace carefully, directing them in lifestyles of artistic creation, contemplation, mental discipline and physical rigor. It was this lifestyle that delayed the emergence of Slannesh. Unfortunately, even in a society that is highly regulated, cultural decadence can slip in and the psychic residue of their hedonistic lifestyles eventually reached critical mass, Slannesh (or She Who Thirsts) was born.
The psychic essence of the Eldar was so subtle and yet so powerful that when it was released and Slannesh was created, the release caused the rupture in space-time known as the Eye of Terror and killed many of the earlier but less powerful Chaos gods. The Eye of Terror's creation destroyed a significant part of the Eldar civilization, adding to Slannesh's power through the absorption of those Eldar spiritual essences. The Eye of Terror is now a focal point by which many of the Gods of Chaos now enter directly into the Universe of the 41st Century.
Slaanesh is the God of hedonism and excess. But this is true in all things, not just carnal pleasures. Those who desire to indulge in the finest culinary delights, the most beautiful artworks, even the most sensual clothing, could all be amongst Slaanesh’s disciples. Just as importantly, Slaanesh is also the God of perfection. The singer striving for the most beautiful song or the warrior who seeks the perfect fighting techniques, both could be devotees of Slaanesh. --Warhammer 40K Wiki