Pendulum of Doom is very rare drop from Uldaman. Patch 2.3.0 reduced the level of mobs in this instance, making either: a.) the item extremely hard to come by since there are only a very small number of mobs in the instance that can drop it or b.) the item unobtainable. GMs have confirmed the item is still in the game though it has been unconfirmed that a single one has dropped since Patch 2.3.0.