The Vile Four (四天王, Shitenō?, lit. "Four Heavenly Kings") are the primary antagonists of Final Fantasy Mystic Quest. They serve as the Four Fiends in the game, and are explicitly referred to as such in the Japanese version. They are servants of the Dark King and attack the Crystals' resting place to drain their energy as part of his plan to destroy the world. The Vile Four are Flamerus Rex of Earth, Ice Golem of Water, Dualhead Hydra of Fire, and Pazuzu of Wind.
It is not made clear how the Vile Four came to be at their current state at the start of the game. Otto tells the player his daughter Norma was stranded in Pazuzu's Tower when the Rainbow Bridge malfunctioned, suggesting that the tower has existed long before the game started, and that the villagers were familiar enough with it to explore it despite being infested with monsters.
What is clear is that some time prior to the game, the Vile Four each sealed a door in the Focus Tower with a magical coin to prevent the four regions of the world from communicating with each other, then attacked the Crystals' resting places and began draining their energy. Without the light of the Crystals, the regions of the world began to experience horrible disasters, and the Vile Four in turn became stronger with their energy.