For all the newcomers to the world of Diablo, here's a short synopsis of the story so far. Renegade angels and demons who grew tired of the eternal war between High Heavens and Burning Hells created Sanctuary, a realm for their offspring, the Nephalem. The Prime Evils, rulers of the Burning Hells, took interest in the Nephalem, since they are stronger than angels and demons; the Lesser Evils saw this as a distraction from the Great War, which led to the banishment of the Prime Evils, Baal, Diablo and Mephisto.
With the help of angels a group of humans imprisoned The Prime Evils - in soulstones. However, Diablo soon escaped his prison through corrupting nearby humans and finally possessing them. Diablo now free from his imprisonment, went on to free his brothers, Baal and Mephisto. A new group of heroes hunted down and slain Diablo and Mephisto however they were too late to stop Baal from corrupting the worldstone. The Archangel Tyrael decided to destroy the worldstone since he feared for the safety of Sanctuary.
Now, 20 years after the destruction of the worldstone, a meteor plunges into the Tristram Cathedral, setting the foundation for the plot of Diablo 3. A prophecy foretold that once the worldstone is destroyed it will break open the gates of Hell unleashing the forces of the Burning Hells; however the invasion never happened which caused Deckard Cain to seek an answer. Blizzard also released a series of videos through the development stages hinting to the overall plot of Diablo. In one of these videos, Leah can be seen staring at the Black Soulstone when she suddenly has a vision of some kind, where Azmodan declares that he knows that the stone is the key and once it is in his grasps he will rule as the Prime Evil of the Burning Hells.
Much of the story of Diablo will focus on Archangel Tyrael and he's influence on Sanctuary. It will also be the first time that Belial - The Lord of Lies, and Azmodan - The Lord of Sin, will appear in the Diablo series and with the return of Diablo confirmed it will presumably be two all-out wars, one will be the ongoing and eternal war between Heaven and Hell and the other war could be between The Great Evils as they battle it out to rule as The Prime Evil. The story is set to continue soon with blizzard announcing the launch date of the game.