It usually isn't alone, so be prepared for a fight. It will spawn when you walk past an archway after you take right turn after the rat gate trap, near Hearthsinger Forresten. The Unforgiven spawn is scripted, so have your party members rest before going any closer to the archway. Some non-elites spawn too, but a mage can take care of them easily. The Unforgiven casts Frost Nova now and then. Very easy fight.
This mini-boss could be a reference to the Metallica song "The Unforgiven".