Doom Engine

When the forces of Chaos launch a major invasion they often build crude siege towers or entreat daemonic aid for more potent engines of war. The smoke-spewing machineries built by Hothgar, the renegade Chaos Dwarf Sorcerer of the Forge, however, are prized beyond all others. These vast war towers are self-propelled, powered by steam and, some say, daemonic pacts. With nigh-impenetrable iron plating, these Doom Engines grind over armies, bastions, and castles alike. They will reach their destination and there disgorge a legion of troops. Luckily such ironclad monstrosities are rarely seen, although none know Hothgar's whereabouts or who might be hiring his services.

The destruction of the walled town of Kurskinstadt and the utter collapse of the Skaven lair-nest of Gribblehook are attributed to the clanking, grinding, and pulverising war towers built by Hothgar. It is rumoured that the renegade Chaos Dwarf has once again been embraced by the powers within Zharr Naggrund.

Source
pg. 426
 * Warhammer Fantasy Battles (8th Edition)