Trebuchets are siege weapons found in the Fields of Slaughter during the Machines of War quest in Diablo III. They are massive immobile artillery of the Army of Sin, raining death down on the walls of Bastion's Keep.
The Trebuchet itself is too massive to damage it directly, and is located deep in the magma-filled pit right in the middle of the map. To disable the siege engine, players need to approach it from the south-west, until the gates open. Inside, two Blood Clan Khazra Maulers and one Blood Clan Shaman guard the controlling device. Destroying the device will cause the Trebuchet to collapse into molten magma.