Many years ago, Pandaemonium rose into the world, and the Jade Passage opened far to the east of Mysidia. Demons from Pandaemonium attacked the world through the passage, until the mages of Mysidia crafted Ultima as the ultimate spell. Using Ultima, Pandaemonium was sunk back to Hell and the Jade Passage closed. The Mysidian mages feared for the world's safety and sealed the Ultima Tome in the Mysidian Tower. Entry to the tower was made possible with a Crystal Rod, which required a Black Mask and White Mask to be obtained. Furthemore, Leviathan served as a guardian for the tower.
The Wild Rose Rebellion's fight against Palamecia is dire enough to use Ultima. Princess Hilda orders Minwu to travel to Mysidia and retrieve the Ultima Tome, but he takes longer than she expected and she sends Firion and his friends to find him. At the top of the Mysidian Tower Minwu sacrifices his life to destroy the seal on the door to the chamber where the Ultima Tome lay.
Ultima is classified as White Magic, as it raises Spirit when used repeatedly. Its damage is not determined by Spirit or Intelligence; instead, Ultima's damage relies on the number of spells the caster knows and their level—the more spells and the higher their levels, the more damage Ultima does. Thus Ultima may do next to no damage, but when taught to someone with numerous high-level spells, it is the most powerful spell in the game, far surpassing Flare and Holy, the otherwise two strongest Black and White Magic spells. Ultima's damage also relies on the level of the user's weapon skills, and as with the spells, the damage depends on how high the skill levels for each weapon are. As with any spell, the player can teach Ultima to any party member but as the Ultima Tome is unique, they can only teach it once. In Soul of Rebirth, the player can only teach Ultima to Minwu.
When Final Fantasy II was originally released on the Famicom, the Ultima spell, which was difficult to acquire, was practically useless. It was initially meant to increase its power relative to the level of other spells the caster had, but due to a bug, the spell did just around 500 damage at the most on a single target. Director Hironobu Sakaguchi has later told a humorous story relating to the bug: When Square tested the game and saw the bug, Sakaguchi asked for it to be fixed, but the person who programmed it replied that legendary stuff that dates back to an age before "proper techniques" would look inferior from present's point-of-view, explaining Ultima's weakness. He reasoned that the struggle to acquire it only to discover it's useless mirrors real life, and thus he was not going to fix the bug. Sakaguchi was irritated by the reply and tried to fix it himself, but the programmer had ciphered the source and Ultima was left the way it was.
- In the cutscene before the battle with the Light Emperor in Soul of Rebirth, when Minwu and the party see a vision of Firion and his allies fighting the Dark Emperor, it is Firion who was taught Ultima, which is maxed out at Level 16. This scene is the same regardless of which party member the player actually taught Ultima to in the main game.