Although 2 of the Prime Evils had been destroyed, Baal was free and out to corrupt the Worldstone and all of humankind. He wanted to control humans and tip the balance of the Eternal Conflict. Baal knew that he needed the Amber Soulstone to do this. Marius had possession of this soulstone. Tyrael had ordered Marius to destroy the stone, but he had instead fled to Westmarch where he went mad and was confined to a sanitarium. Baal swept an inferno through the sanitarium and took back the soulstone.
While Baal was imprisoned in the soulstone, the soulstone itself had become corrupted and destructive. All Baal had to do is melt the soulstone onto the Worldstone to corrupt the crystal.
Baal marched on to Mount Arreat decimating villages along the way. Those who died transformed into demonic soldiers making them soldiers of Baal. With every death Baal had caused in Sanctuary, the soulstone grew fiercer. This way, the soulstone would be even more destructive when melted onto the Worldstone.
At Mount Arreat, the barbarians fought fiercely against Baal but were unable to prevent him from capturing their capital of Sescheron. All that stood in Baal’s way now was a fortress nestled in the mountain’s foothills. The Elders grew desperate at the sight of Baal and sacrificed themselves to create a protective barrier around Harrogath. At this point, the brave warriors who defeated Diablo and Mephisto arrived at Harragoth. An assassin and druid had now joined the heroes, improving their chances of victory.
As they chased the demon, Baal breached the Worldstone’s secret chamber within the bowels of the mountain. A great battle ensued as the warriors caught up with Baal and barely defeated him. To their dismay, Baal had already fused the shard with the Worldstone. Tyrael arrived and saw that soon, all of mankind would turn evil and violent. In a desperate situation, Tyrael then used his sword to destroy the Worldstone. It shattered with a horrific explosion. This very explosion annihilated Baal’s being and sent him into the abyss with the other Prime Evils. Rumors have it that Tyrael was also annihilated and thrown into the abyss.
leaders of hell had been defeated but Azmodan, the Lord of Sin, and Belial, the Lord of Lies were not. Cain believes the end is near and that these 2 will play a vital role in it. It is also disheartening that Tyrael was alone in his acts to protect Sanctuary. Where the rest of the High Heavens were is a mystery.