The Crimson Moon is a major plot element from Final Fantasy Crystal Chronicles: Ring of Fates. It is an evil influence on the world and its followers serve as the primary antagonists of the game.
The Crimson Moon was present during the "Golden Age" of the world, thousands of years before the events of Final Fantasy Crystal Chronicles. During this time, the four tribes lived in peace and were governed from the city of Rebena Te Ra. The Great Crystal protected people from the "malefic glow of the blood-red moon".
The Moon's influence can corrupt crystals, turning them red. Animals with corrupted crystals in their bodies become powerful and violent, but the crystal also serves as a weak point. Yuri and his companions encounter many such monsters over the course of the game. It is also thought that the moon causes monsters to appear generally.
People who worship the Moon are called Lunites or "moontouched". The Moon's nature is greed, hatred, and other traits that lead to evil. Because it directly opposes the Great Crystal's nature, moontouched people are unable to use crystals. Lunites led by Galdes and Cu Chaspel took over the Crystal Temple some years before the start of the game.
The Moon can influence people who do not actively worship it; one can become moontouched by giving into selfish or evil desires. Latov and Aleria feared that the Moon would corrupt their children with revenge if they knew of the Lunites' crimes, so they hid the truth of Aleria's abduction from them. Galdes and Cu Chaspel were able to seize control of the Temple easily because the clerics had already been corrupted by their petty scheming for money and power.
The Moon was created while the earth was still forming. The crystals repelled all that was evil and ejected it into the cosmos. Although this protected life to a great degree, the mass became the Moon and remained in orbit, able to corrupt living things. At the end of Ring of Fates, Yuri and Chelinka create a new world where the Moon is still present, but it is not and is said to never have been evil.