The story of Diablo 3 plays 20 years after the happenings of the second part of the Diablo series.
The evil triumvirate, composed of the three prime evils Mephisto, Lord of Hate, Diablo, Lord of Terror and Baal, Lord of Destruction were defeated by countless brave warriors.
In a last, grim battle, high above on the icy summit of Mount Arreat, where the Worldstone was located, Baal, Lord of Destruction failed to finish his mischievous plan, and was overwhelmed by a handful of great heroes.
But the success had a bitter taste.
The worldstone, an ancient relic of unknown power was destroyed by the magical blade of Tyrael himself, after Baal corrupted the worldstone through his foul presence.
No one ever understood the powers that lied within the worldstone, so consequences of its destruction would be incalculable.

Leaving the corrupted worldstone intact, was no option either, it would have allowed all demonic creatures from hell free access to Sanctuary.
It would have led to the end of the world.

Two decades later.
The great suffering of the past is only known to oldest of the inhabitants of sanctuary, and even the last remaining witnesses of the battle 20 years ago, try to avoid to even speak about the happenings that led to destruction of the worldstone.

But the evil is not defeated yet.
Now 20 years later, rumours about unspoken horrors again roam across the lands, and wanderers whisper about strange creatures seen near the rebuilt Tristram.
Deckard Cain, the last of the Horadrim, and also the only survivor of Old Tristram has spent years of research after the destruction of the worldstones, to find answers to the open questions that burned deep within his soul.
Where are the other prime evils, Azmodan, Lord of Lies and Belial, Lord of Sin? Are they planning some dark deeds again? What happened after the fall of the worldstone? Why didn’t hell sent its demonic army through the portal to conquer sanctuary?

Tyrael seems to be a key figure in all this riddles.
We shall find out…