Tal Rasha was a powerful Horadric mage. He was originally part of the Horadrim who pursued Baal, and when Baal shattered his Soulstone, Tal Rasha offered to contain the demon within his body. Tal Rasha was bound to a pillar in one of the seven ancient tombs in the desert, and the shard of the Soulstone was jammed into his chest, releasing the Lord of Destruction's spirit into his body. The tomb was then sealed.
Tal Rasha and Jered Cain led the Horadrim, who captured and imprisoned Mephisto in one of three Soulstones given to them by the Archangel Tyrael. When they attempted to imprison Baal within his Soulstone, it was shattered by the Prime Evil. In order to contain Baal's spirit without the whole Soulstone, Tal Rasha volunteered to use his own body as an extension of the Soulstone. The largest shard of the shattered Soulstone was impaled in Tal Rasha's chest, and Tal Rasha allowed himself to be possessed by Baal. Jered Cain and the other Horadrim imprisoned Tal Rasha's body in a secret tomb in the Canyon of the Magi in Aranoch, where Tal Rasha's and Baal's spirits were to have wrestled with each other for all eternity. The operation was overseen by the Archangel Tyrael.
The tomb's location was forgotten with the sands of history until Diablo, in possession of the body of the Dark Wanderer, and Marius found Tal Rasha's tomb many years later. Tyrael arrived in attempts to prevent Diablo from freeing his brother, but he never saw Marius closing in to remove the Soulstone from Tal Rasha's chest until it was too late. As a result, Baal was freed from his prison while still leaving him in possession of Tal Rasha's body. According to Drognan and Fara, this gave Baal unrestricted access to Tal Rasha's knowledge and powers.