Tarrasque

Six short legs like a bear's, the body of an ox under a spiked shell, a lion's head, a horse's ears and the face of a bitter old man, with a tail that ends in a sting. It came out of the river and ate everyone who went near it for a very long time, and every knight sent against it stayed. What finally worked was a woman who walked up to it alone with holy water and a cross, and it stood still like a sheep, and she led it into the town on her own belt. The townspeople killed it while it stood there tame. She wept for it, and forgave them, because they had suffered so long.

Rush: bludgeoning, melee 20 ft, +7 to hit, 3d10+6, twice each round from a body that does not slow down for walls. Sting: piercing, melee 20 ft, +7 to hit, 3d8+6, its third attack, and the wound does not close on its own.

Nothing Stops It: DR 4, and it is unaffected by fear, pain, exhaustion and morale. Armies have been used and are in the record as having failed. Stilled: holy water cast on it, with the cross shown, stills it completely - no save, no roll, no contest. It stands like a sheep and can be bound with any belt or cord and led anywhere at a walk. It does not resist and does not resume. A creature may then kill it where it stands, easily, and the DN should let the party feel exactly what that is. Reduced to 0 HP in open combat, which has not been done, it dies.