The Meteor

In the Diablo 3 cinematic trailer, twenty years after the destruction of the Worldstone and the defeat of the Prime Evils, a blazing meteor is seen falling across the sky, landing abruptly upon the New Tristram's Cathedral. Leah and Deckard Cain were at the cathedral at the time the meteor struck crashing through many levels down into the basement. It presumably crashed to the bottom of the basement where Diablo was originally defeated in Diablo I.

In the Blizzcon playable demo, Captain Rumford says a meteor had struck the cathedral and it has awakened evil within. He asks you to investigate.