Koothulu is a god of destruction who travels inside a comet-like shell. He is the main villain of Mario and Luigi: Comet Quest, although this is not revealed until the game's end. His lust for destruction causes him to engage in an everlasting ritual where he destroys the planet Mechior every one thousand years. The Ancient Minister created the subspace bombs in response to Koothulu's wrath. However, the Minister revealed his true intentions after being defeated by Mario and his party, and took the gang to Melchior to battle Koothulu. Koothulu was defeated, thus causing him to leave Melchior forever.
- Koothulu's name comes from Cthulhu, the evil god of H.P Lovecraft's mythos.