A Soulstone is a shard of the Worldstone specifically molded to contain the soul of a powerful being. There are only four known to have existed; three were used to capture the Prime Evils during the Dark Exile, and the Black Soulstone was made by Zoltun Kulle after the Three had been captured.
The soulstones were created by the archangel Tyrael in order to contain the spirits of the three Prime Evils when they were exiled to Sanctuary. Three were made, each in a distinct color for a specific Prime Evil:
The soulstones were given to the Horadrim, specifically Zoltun Kulle who was tasked with carrying them. The stones were attuned to their respective Prime Evil and allowed the Horadrim to track the demons across Sanctuary The Horadrim were eventually successful in capturing all of the Three, although Baal proved to be problematic to capture. During the battle between him and the mages, his desctructive nature shattered the Amber Soulstone, leaving it broken. Zoltun Kulle however theorized that the stone might still function if completed with a living human body. Tal Rasha volunteered to complete the , and though his mind was eventually taken over by the Lord of Destruction, he kept Baal trapped within until his brother Diablo came and released him.
The soulstones did not turn out to be the perfect prisons Tyrael had imagined. During a conversation with Izual in Diablo II, it is revealed that Izual told the Prime Evils how the soulstones worked and how to corrupt them, which the Prime Evils all did. Mephisto worked his way into the minds of the Zakarum High Council, eventually corrupting all but the pious Khalim to serve him, while Diablo similarly reached out to both Leoric and Aidan from beneath the cathedral in Tristram. Baal would perhaps similarly reached out to surrounding mortals had he not been imprisoned far out in the desert of Aranoch. As it were he did get control of Tal Rasha, one of the most powerful mages in recent history.
Fate of the Soulstones
Eventually all three Evils were able to roam across Sanctuary freely despite still being tied to their soulstone prisons, however that did not save them from their eventual demise. Both Mephisto and Diablo were defeated and their soulstones shattered at the Hellforge, a powerful artifact in the Realm of Destruction capable of destroying anything no matter how powerful.
Baal's soulstone was also destroyed, but not on the Anvil of Annihilation like the other two. Instead, it was fused into the Worldstone by Baal himself moments before his death, thereby corrupting the Worldstone and forcing Tyrael to destroy it.
The Black Soulstone
After the Three were imprisoned, Zoltun Kulle was left a hollow man of his former self. He became obsessed with the soulstones and tried to fashion one for himself, the Black Soulstone, which would be able to harness both demonic and angelic beings. What happened to the stone is at this time unknown, but it appears Leah comes in possession of it during the events of Diablo III and that it is the driving motivation for Azmodan's invasion of Sanctuary.
- Cain, Deckard., Book of Cain, page 88.
- Cain, Deckard., Book of Cain, page 89.
- Cain, Deckard., Book of Cain, page 92.
- Cain, Deckard., Book of Cain, page 121.
- Cain, Deckard., Book of Cain, page 127.