The Fall of the Eldar occurred over ten thousand years ago and ended the Eldar's supremacy in The Galaxy.

Why did the Eldar empire fall?

The fall of this empire came not from the actions of another species, but literally from the conflicts that lay within the Aeldari themselves. Aeldari technology had advanced so far by the height of their empire that little or no work was required by individual Aeldari.

What happens to Eldar killed by slaanesh?

With the sudden deaths of so many Eldar, the old gods were drastically weakened. Slaanesh took advantage of this weakness and destroyed them, assimilating them within itself. ... However, among the death and destruction, two of the original Eldar Gods survived.