The Chalice of Eternity is in the northernmost cavern of the Waterfall Dungeon. The chalice is not considered an item, because players cannot actually gain possession of it.
Properly using the chalice is the climax of the Waterfall Quest. Warning: Players who attempt to "Take treasure Chalice of Eternity" (right click option) without following the proper procedure in the quest will be washed out of the cavern. They will also lose Glarial's amulet and Glarial's urn. (Both can be retrieved from Glarial's tomb. The amulet is required to enter the dungeon; the urn is used with the chalice in the quest.)