Treasure Hunter is a light armor set. The 5-piece set bonus adds 100 Spell Damage and lets you gain Major Prophecy at all times, increasing your Spell Critical rating by 2191.
The Treasure Hunter set is part of the base game. It drops in Volenfell in the Alik'r Desert. Each set item is bound on pickup. You can acquire light armor, weapons and jewelry of this set in the Mercenary motif style.