Andariel, the sadistic Maiden of Anguish, has left a great legacy upon the Western Kingdoms. But her story begins long ago.
Belial and Azmodan, growing tired of constantly vying for the favour of Man, masterminded a scheme which would become known as the Dark Exile. Allying with Andariel and Duriel, the other Lesser Evils, they made war upon the Three, and finally, after a battle that killed over a third of Hell's forces, the Prime Evils were defeated, and banished to Sanctuary.
But Belial and Azmodan began to argue over who had dominance, and plunged Hell into another civil war that lasted for ages.
Duriel and Andariel, however, sensed the rise of Diablo in Sanctuary after centuries of slumber beneath Tristram. Seeking to finally free themselves from a pointless civil war, that did not serve their unholy mission of chaos, they offered their assistance to the Prime Evils. They overthrew Belial and Azmodan, and put the allegiance of Hell back towards the Three.
However, their jobs were not done yet. Diablo was greatly vexed by the Order of the Sightless Eye, whose sisters had destroyed many of his forces in Tristram. To avenge himself, he set Andariel the task of taking their Monastery, which served as a guardian of the path to the East, and corrupting their sisterhood.
Andariel made her home beneath the monastery and wove vile spells of mental domination, permeating the minds of the sisters. Blood Raven was the first to be corrupted, and she turned many sisters against her rival Kashya, before the untainted sisters saw the corruption that was at hand.
Andariel worked to transform the Monastery into an outpost of Hell to eternally guard the Rogue Pass, but was, fortunately, stopped by a group of heroes. However, her taint has left its mark upon the Sisterhood.