If you played Diablo 3's beta you're famillar with the character of Leah. She is the star of the dramatic cinematic video from that debuted at Blizzcon 2011 called "Black Soulstone." In it Leah, who is looking for her uncle, is greeted by a not-so friendly demon. Let's call him Diablo why don't we?
"You thought you were so clever. That you'd out wit us all. One by one our brotheren fell into your trap. but not me. I defy you. I know the black soulstone is the key and it shall be mine.
Key elements were taken out of this video to prevent any spoilers but from what I gather, that guy with the glowing eyes wants the black soulstone.