Source: Dungeons & Dragons
Location: Wherever it damn well pleases to be
Threat Assessment: 6. The ruler of evil dragons, this dragon has the heads of all the "color" dragons (red, white, blue, black, green) with the breath weapons of each type (flame, cold, lightning, acid, poison gas). Tiamat also has great mystical power and is nigh invulnerable.
Limitations: Greedy. Not Subtle. Other than that, run.