Crystal Mallet

Crystal Mallet is an quest item which is needed at the fourth floor of Watcher's Keep (dungeon). It's needed to break the glass of the Machine of Lum the Mad and to free Carston from his life sustaining cage in which he is trapped for more than ten years. You need to swing this six times precisely, no more and no less or otherwise nothing will happen. It has a weight of 1 lbs.

It can be found in the same area as the machine, but it's stored in a safe behind the doors of the oil puzzle. This item appears in Baldur's Gate II: Throne of Bhaal.

In-game description
"This finely crafted hammer appears too delicate to be of any real use as a tool - unless, of course, it has some magical application."