Basic Information
Tal Rasha is one of the most fleshed out NPC's in the Diablo universe, although the player never meets him. He also inspired one of the few most powerful sets of Diablo II items which have Display Auras: Tal Rasha's Wrappings.
Diablo Lore
Tal Rasha was the leader of the Horadrim during the last Sin War. When the Prime Evils were exiled from Hell into the Mortal Realm by the Lesser Evils a group of mages banded together to defeat them and bind them to Soulstones which were provided by the Archangel Tyrael.
While the mages were fighting against Baal, the soulstone they were going to imprison him in was broken. Tal Rasha decided that the only way for them to bind him was to first capture his essence temporarily in a soul stone, and for Tal Rasha to push the soulstone into his own body. This they did, and sealed him inside his tomb to struggle for eternity against Baal.
This Act was later shown to be mere hubris, since Tal Rasha was utterly corrupted by Baal, and was later easily freed by Marius in the Cinematics.
Other NPC's opinions
Horazon lauds Tal Rasha and calls him "the foremost" of all of the Horadrim. Similarly on the eve of the final battle with Baal-cum-Tal Rasha, Deckard Cain makes clear his respect for the wizard:
"Remember this: Baal once possessed Tal Rasha, one of the most powerful of the ancient Horadrim. Your battles with Diablo and Mephisto will pale in comparison to your battle with Baal. The Lord of Destruction, aided by Tal Rasha’s knowledge… Ungh, the mountain itself will tremble when you clash!"
Two other Act II NPC's are not quite so supportive of Tal Rasha's decision however. Fara tells us that "his was a selfless act, although perhaps foolhardy." and Lysander, seeing the evil which plagued Lut Gholein muses that Tal Rasha's battle against evil was lost.