Moon Man is a huge chunk of moon rock with a face, about 4 metres squared (sphere) that is incredibly evil. Every months for one night he comes to earth and murders and tortures as many people as he can. He is impossible to kill, except for the cat gods. Moon man also has babies called Moonlings that multiply. Moon Man can't stay in the day, so if you capture him at night and keep him in a prison cell, when it becomes dawn he has to go back to the moon and can travel through walls. The murders of Moon Man can be tracked back to viking times. The mythology of werewolves on full moons were based off Moon Man's killings.
Moon Man was originally beaten by the cat gods, and sent into a coma in the moon along time ago, but was reawakened by the first rocket on the moon (that actually wasn't Apollo 11, but another man who's theory people laughed at, so he went by himself to prove it, but no one ever found out he had actually travelled to the moon first.) The first man on the moon (not Neil Armstrong or any of the people on Apollo 11) was kidnapped by the newly awoken Moon Man's Moonlings and was mutated, to become The Man on the Moon.
Moon Man also becomes more powerful and even more sadistic and evil on full moons. And if it ever became a blue moon he would be able to destroy the human race.