N'Zoth escapes in 8.3 and he comes back with a vengeance.
While the Alliance and Horde were busy fighting each other, N'Zoth was secretly building armies while manipulating leaders of factions to do his bidding.
He will raise the Black Empire back to it's full glory. Azeroth will be in ruins after we return to find out it's been overrun by the Naga inquisition.