Duriel can be found in one of the seven tombs. Each tomb is represented by a unique symbol, whose locations are not random and are the same for each game. The correct sign, and therefore the correct tomb in which to look for Duriel, will be shown in your Quest Log. You must have the Horadric Staff to open the tomb for the first time.
Quest Reward
Quest drop if a player needing the quest kills Duriel and access to Act III.