Three hundred years ago Chaos appeared in the world when humans nurtured darkness in their hearts. He was sealed away by the dragons of Spelvia. Fifteen years ago Chaos and his servants were released and caused turmoil throughout the world. Rolan from Spelvia was sent to stop his minions, but was corrupted himself by Lucifer and gave up his quest, leaving them free to carry out their plans.
Chaos is not mentioned until the party defeats Rolan in Spelvia. The Heroes of Light are shot back into the past when Chaos is released, and are forced to defeat his servants. The heroes correct the mistakes made in the past, and obtain the legendary weapons. After they defeat all of Chaos' servants, he opens the entrance to the Star Chamber, which will eventually envelop the entire world in darkness, and revives his servants within. After the heroes defeat his servants and regain their crowns, the party meets Chaos in the dungeon's final level. Chaos battles the party twice, but is defeated.
After defeating his second form, the party is about to give up, but their allies encourage them to not give up and give them the power to stand again. The party defeats Chaos, but before he dies, he warns he will always exist as long as there is hatred in the hearts of people.
Chaos is fought thrice as the game's final boss: The first phase is a fight against Chaos's initial form, the second phase is large gray monster, and the third form is the Chaos of Space and Time.
The battle theme for Chaos consists of three parts: "A Strong Enemy" for his first form, "The Demon King's Servants" for his second form, and "The Final Battle" for his final form.
Behind the scenes
Chaos uses many special attacks that belong to previous final bosses from the series: Tsunami, Blaze, Earthquake, and Tornado are attacks used by Chaos from the original Final Fantasy. Lightning is a special attack used by the Cloud of Darkness from Final Fantasy III and Big Bang is an attack used originally by Zeromus from Final Fantasy IV.
The speech Chaos makes before dying is an allusion to the final words Zeromus speaks before dying in Final Fantasy IV.
Chaos refers to the void state preceding the creation of the universe in Greek creation myths, or to the initial "gap" created by the original separation of heaven and earth. Chaos also refers to a state of disorder, disharmony, dissonance, lawlessness, and unpredictability.