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Once the heroes reach the throne room inside the World of Nil, they come across Elgo who reveals himself to have been the Emperor of Avalon all along. He harnesses the power of Nil to become immortal, but after the party defeats him, he is consumed by the power and turned into the monstrosity that is Chaos Elgo.
The creature escapes between worlds through a dimensional portal with the heroes in pursuit. They face off with Chaos Elgo in the final battle, ultimately defeating him and saving the world.
The battle starts off with Chaos Elgo casting Grand Wall, which deals roughly 3,000 damage to a single party member. It will also use Eclipse to deal about 1,000 damage to the party and randomly add Curse. Sometimes Chaos Elgo will counter with Gravity. After dealing a sufficient amount of damage, Chaos Elgo will transform into its second state and cast Dark Flow, instantly killing the party. After a short cutscene plays, the heroes will be revived with their buffs intact. It is possible for Chaos Elgo to cast Black Hole, nullifying the status buffs.
From this point on, Chaos Elgo will switch between three forms: the default form which uses physical and non-elemental attacks, the Dark form which uses Black Magic, status ailments and Dark-elemental attacks, and the Light form which heals and casts buffs on itself while unleashing Holy-elemental damage. The light and dark forms are resistant to their respective elements.
The boss can also use 10000 G's, which will reduce the entire party to single digit HP. When low on health, Chaos Elgo will begin to use Big Bang, which will deal around 4,500 damage to the party along with Sap.
It is recommended the party be level 70 or above before attempting this battle. The battle will be long, so the player should ensure they have a good supply of Dry Ether and Elixir items. The Argy summon is helpful to counteract Chaos Elgo's Black Hole ability.
The party should begin by casting as many status buffs as possible, then unleash the strongest attacks available. If Chaos Elgo casts Black Hole and nullifies the party's buffs, it is recommended to recast Shell and Haste.
The song that plays during the battle with Chaos Elgo is titled "Inside the Swirl of Chaos" and was composed by Naoshi Mizuta.