Tal was originally the leader of the Horadrim’s chase of the Prime Evils during the Sin Wars, and successfully led his brethren into defeating and capturing Mephisto, prisoning him in a Soulstone given to them by Tyrael, the Archangel. They followed Baal to the Western continent and managed to defeat him in the Aranoch desert, somewhere near Lut Gholein, but at a price. Baal managed to shatter the Soulstone meant for him before he was overcome, and Tal Rasha volunteered to help contain the demon, forever battling with his spirit.
What became of his body and personality isn’t widely known, and exactly what the background is for his equipment, Tal Rasha’s Wrappings is also an unknown (this is one of the most detailed item sets in the world of Sanctuary). However, if you are interested in this powerful character of the world of Diablo, do take a look at the DiabloWiki article: Tal Rasha.