Tarrasque

The Tarrasque is a pure engine of fucking destruction. Dreaded by adventurers for its high hp value and its obscenely high regeneration ability, it is known to rampage across nations eating everything. It cannot fully be killed through normal means; therefore, A player (or group) lucky enough to reduce a Tarrasque to negative 10 hp will then have to use a wish or miracle spell to completely destroy it, with only a 50% chance of success. if unsuccessful it will wake up again x years later and fuck more shit up. Also, no matter how big your army is (assuming your adventuring party had enough cash to recruit mercenaries), the Tarrasque's fast metabolism will allow him to produce such powerful flatulence that all your troops will die by suffocation in just one turn. The only thing that can really stop him is Chuck Norris on a bad day.