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A Demonic Ballisa

Demonic Ballistae, Dread War Machines are Super Unique constructs found in the Fields of Slaughter during the Machines of War quest in Diablo III. These are the siege engines of the Army of Sin, raining death down on the walls of Bastion's Keep.

Visually, they resemble Catapults from Diablo II. Their ammunition is conjured immediately before the shot, and each Ballista is powered by a huge spine attached to it, as well as demonic rune circle. 

Players must destroy all three Ballistae to succeed. They spawn randomly at six possible locations. Their firing animation is purely cosmetic, for a Ballista cannot fight back, however, all of them are surrounded by Mine Fields and guarded by demonic creatures. Destroying a Ballista (which is not hard, due to their average durability) also attracts a group of foes.