Zemus

Zemus is the mastermind behind Golbez's plot in Final Fantasy IV. Wishing to destroy the planet, he manipulates Golbez into carrying out his will in hunting down the Crystals. Fusoya reveals that Zemus is the only Lunarian who refused to go into stasis until the time when Lunarians and Earthlings could live in harmony. Even though Golbez and Fusoya seemingly vanquish him, Zemus comes back to life as Zeromus, the monstrous embodiment of his hatred and evil. Cecil and his friends use a Crystal to reveal the true shape of Zeromus, and manage to slay him.

Final Fantasy IV
In Final Fantasy IV Zemus does not appear until moments before the final boss fight, although his presence is felt throughout the entire game. As Cecil and his brother Golbez are both sons of the Lunarian Kluya and an earthling woman, Zemus could theoretically have power over either of them. Cecil lacks the evil in his heart that Golbez has. Throughout the game, Zemus uses his telepathic influence over Golbez, using him as a tool of evil. He also manipulates Golbez's sorcery powers to take control of Kain.

Also tools of Zemus are the Four Fiends of the Elements, who are merely followers of Zemus, not victims of his telepathic powers. After Kain restrains from killing Cecil, Golbez (being controlled by Zemus) sends the Four Fiends to do the dirty work of killing the heroes.

Zemus's final tool of evil is Dr. Lugae. Though it is never said whether Dr. Lugae is being controlled by Golbez or not, he is the self-proclaimed "brains behind Golbez's operation" who works in conjunction with the Fire Fiend Rubicant. Dr. Lugae is soon killed by the party in the Tower of Babil.

Cecil first learns of Zemus after meeting the Lunarian Fusoya on the moon. Fusoya reveals that Zemus plans to use the power of the Crystals to call forth the Giant of Babil, which will destroy all life on Earth. Cecil and Fusoya return to Earth in hopes of stopping the Giant's summoning. However, they return too late. They go inside of the Giant and destroy its core, as well as the Four Fiends. Golbez storms in enraged, but Fusoya uses his power to release Golbez from Zemus's control. This also ends Golbez's control over Kain.

When Golbez realizes what has happened, he and Fusoya return to the moon and kill Zemus using the White Meteo spell. Though Zemus dies, the evil inside of him takes a physical form, Zeromus. Golbez and Fusoya attempt to use the White Meteo spell again, but to no avail. To weaken Zeromus's powers, Golbez tries to use the Crystal. However, the small amount of evil still inside Golbez's heart does not allow the Crystal to work. Cecil, pure of heart, then uses the Crystal and defeats Zeromus, returning peace to the world.