This terrible artifact was constructed by the Dwarfs in ages past as a weapon of last resort. There has only been one recorded instance of Townkiller being used in battle, a situation that quickly revealed the device's terrible potency.
Townkiller is a forty-foot long juggernaut of metal and stone. Its worm-like body is made up of a dozen segments of magically hardened obsidian laced together with a steel matrix. Dozens of multi-jointed, adamantite limbs sprout from these segments, each five or six feet long. At the forward end a ring of gleaming diamonds surround a circular maw lined with dozens of smaller limbs and dagger-like adamantite fangs, the whole forming a funnel-like opening. The bottom of this funnel glows with white-hot energy. At the rearward end Townkiller tapers to a small, spout-like final segment.
The artifact radiates Legendary protection, animation and evocation magic along with Epic divination magic. Discovering the mechanisms and magics that drive the device is a Legendary test of artifice, and requires access to a Superb quality library of Dwarf-lore.
Townkiller can also be commanded to simply move. It travels to its destination at Great speed, leaving a tunnel lined with melted stone in its wake. The device can easily travel 50 miles per day.
The artifact is effectively indestructible. It's adamantite and magic enhanced body shrugs off physical and magical damage with ease. Should someone accomplish the Legendary task of causing physical damage, the magical matrix encasing the device can heal any damage suffered at the rate of one rank per week. The GM should determine a unique method of destruction as suits their campaign setting. A good place to start might be extreme cold, such as the breath of a white dragon or a trip to a frozen elemental plane.