Tal Rasha was a very powerful Horadrim mage. He was originally part of the group that hunted down Baal. However, Baal shattered his soulstone so that it could not be destroyed. The Archangel Tyrael suggested that they house the spirit of Baal within of the Horadrim's bodies. Tal Rasha offered to contain the great demon within his own body. Tal Rasha was bound to a pillar in one of the seven tombs in the Canyon of the Magi and the shards from Baal's soustone were shoved into his chest. The tomb was sealed and its location was concealed.
In Diablo II the Dark Wanderer and Marius find Tal Rasha's tomb. Tyrael stops the Wanderer from reaching Baal, however, Marius ends us removing the stone from Tal Rasha's chest and releases Baal. He is still trapped within Tal Rasha's body though. Allegedly this gave Baal access to all of Tal Rasha's knowledge. At the End of Act V, Tal Rasha's body is destroyed.
A set is also named after him, Tal Rasha's Wrappings.