Tyrael falls. The dead rise.
You meet an injured Cain in his room at his house. He begs you to rescue Leah. You run up to the cathedral and find Leah in a running battle with the skeleton king’s minions blasting them with magic. You help her fight off the skeletons and she demands you help her find Cain. You inform her of his whereabouts and she rushes back to town. You get the crown and Leah follows you after stabilizing the wounded Cain with her mysterious power. You get to Leoric’s room and find a door that can only be opened by his bloodline. She opens the door, you knock off the king and you rescue Tyrael.
Collect sword pieces. Pop back to Tristram. You find Belial standing over the sword, stranger and Cain. Raging battle ensues, Leah and Cain work together to injure Belial while you fight off minions. Cain is gravely wounded, but recognizes the hilt of El’druin. Reforges the sword, taking the last of his life energy. Instead of letting him unjustly die, he gives El’druin to Cain. This heals Cain, Tyrael gives his warning and dies.
You chase Belial to Leoric’s manner, where he’s attempting a ritual to resurrect Diablo. You destroy the altar, but this drops you into a giant pit with the Butcher. Trickery. Lies. Traps. More fitting of the Lord of Lies. You rescue a prisoner that tells you he’s from Alcarnus, then dies.
You arrive at Caldeum, fight to Alcarnus through all of Belial’s cultists. Cain begs you not to go any further until you’ve figured thing out. You ignore him. You get to Alcarnus and slaughter everyone, including Belial. Oops. It’s all been a major illusion and you’ve just killed everyone that was holding out against the Lord of Lies, the last survivor tells you that he’s taken over Caldeum as the Prime Minister. Ha, ha. Joke’s on you. Cain stays to bury the dead and search for survivors. Leah is beside herself with grief.
Cain discovers that Belial is actually attempting to bring back the Primes with a shard of the Worldstone, but this time to combine them into himself as the Prime Evil. He comes up with a plan to resurrect an old Horadrim to try and help them, since he was the expert on the Soulstones, but driven to hatred of all things by Mephisto.
You pop Kulle together and you stomp off to destroy Belial. Belial drops a line about recognizing Leah. He’s almost completed the ritual, becoming Super Belial and destroying half of Caldeum. You kill him and Kulle helps you contain the power in the Worldstone shard. (Look ma, no gaping plot holes!)