The Smith is a powerful demon and one of Andariel's and Diablo's goons.
He guards the Horadric Malus, in the Barracks of the Rogue Monastery. When Andariel's manipulation overthrew the structure of the Sisterhood, a great struggle between the followers of Blood Raven, corrupted rogue captain and villainess, and Kashya, leader of the Rogue Army and heroine, erupted. The Rogues were forced to give up their ancestral home and the Monastery, which later gets filled with hungry demons. Most of the evil creatures are deadly but The Smith, who guards the Horadric Malus is really mean.
Apart of the Tools of the Trade quest given to the heroes of Diablo 2 by Charsi, barbarian blacksmith and heroine, is to recover the Horadric Malus for her, which is guarded by The Smith. The Smith is a brutal craftsman, who uses the Horadric Malus to create weapons for Diablo's lackies and his dark hammer and melee attacks make him a strong fighter. But he's killed by the heroes of Diablo 2.