Andariel, also known as the Maiden of Anguish, and the other Lesser Evils once overthrew the three Prime Evils - Diablo, Mephisto and Baal - banishing them from Hell to our world according to ancient Horadric texts.
Background
During the demon war some 20 years ago, she was acting on behalf of Diablo to prevent anyone from following him eastward to Lut Gholein and Tal Rasha's Tomb, meaning the forces of Hell were once again aligned behind Diablo and his Brothers.
Whether or not this was a mutual agreement, or a case of the Lesser Evils beaten into submission is unknown.
Andariel was slain by the Barbarian hero (and possibly others) as they moved towards the Aranoch desert.
Diablo II Andariel
For more information about Andariel's stats and tactics against her, please see Diablo 2 Wiki: Andariel.