Blizzard reclaims its place as the premier developer of awesome CGI sequences with the release of a new Diablo 3 trailer, which was revealed at the Spike Video Game Awards travesty.
Presumably used as the intro for the game, the Diablo III trailer takes place in the form of a book with the events of the second game playing out throughout its entirety, until Leah awakens from her nightmare.
She speaks to Cain, who appears to be performing translations of the Horadric prophecies. He intends to alert the world of the coming darkness, until a meteor strikes the Cathedral at Tristram.
All of this happens prior to the start of the game, which is when the player comes in and goes to rescue Cain, who becomes trapped in the ruins.