Archimonde, we assumed, was dead. But he's show up again on Draenor, and it's left a lot of people confused as to how he could have been dead, and suddenly is not. People speculated that just as there are infinite versions of Draenor, there are infinite versions of the Burning Legion as well. That's where a tweet from Alex Afrasiabi comes in.
The fallen Titan unleashed the Burning Legion and sent them across infinity, devouring worlds like cosmic locusts, rendering them lifeless. A nearly unstoppable army of demons under his banner finds worlds and unmakes them, and so far only one planet has held out against the onslaught of Sargeras and his Burning Legion. But who is Sargeras? What motivates this being? What drives him to seek the unmaking of the entire universe?