After the great war between light and dark, forces of darkness were banished from our world by army of king Leonard. The time has passed since that great victory. Now king Amadeus, grandson of Leonard, reigns over the humankind. But not all of the demons were sealed. The demon called Doppelganger escaped due to his unique ability which allowed him to turn into the last devoured living creature.
The story begins when Doppelganger decides to break the seal, and release his brothers.
Refs: ILKke's "The Tower" and Diablo2 screenshots :D