The opening cinematic sets up the story for Reaper of Souls, which focuses on the character of Malthael, the fallen Archangel of Wisdom who was last seen at the end of Diablo 2: Lord of Destruction. In Reaper of Souls, Malthael has returned as the Angel of Death and has stolen the Black Soulstone. It is up to the player, then, to track down Malthael and reclaim the Soulstone.
Here is the trailer:
