It can be found by going into the Kalphite nursery, then taking the tunnel, then entering the room that you fell into during the quest. There is a crack in the floor that can be mined with level 80 Mining (can be boosted), which will open a fissure. The was and a Scabaras mask will be in the room behind the fissure.
Since 5 June2017, it can also be used as a cosmetic override called Pantheon (Scabaras) Was. The override is a wand override listed under daggers in the Wardrobe interface.