Prophecy of Demise

"Though will it come to pass that the firstborn son of noble blood shall rise to power. The child will be learned int eh darkest arts and he will raise an army of terrible beasts. Thus will the Dark King fall, slain by neither blade nor arrow but by a sorcerous power of darkest magic and so shall his body be consumed in the flames and for all eternity burn."

The Prophecy of Demise is a many-parted portent that speaks of how the Elves as a race will inevitably fall.

As Aenarion drew the Blade of Khaine, Caledor was gifted with a prophecy that spoke of the tragedy that would befall Elvenkind. Part of the prophecy talks of a great warrior cast from his home by a sorcerer, and Malekith believes it is he to whom the prophecy refers. As a result, male Dark Elf wizards are regarded with disdain, fear, and superstition, and they cannot be admitted to the Dark Convent. There are those in Naggaroth, however, who will employ such sorcerers to avoid owing a debt to the Convent of Sorceresses.

Source
Warhammer Armies: Dark Elves (7th Edition) -- pg. 62