|First Appearance||Sonic Adventure|
Chaos is a Chao that was mutated by the power of the Chaos Emeralds and acts as their guardian. It appears liquid-like in form and can change it's shape depending on how many Chaos Emeralds it has absorbed.
Before the events of Sonic Adventure the Echidna tribe tried to steal away the Chao Emeralds, Chaos absorbs all the negative energy of the emeralds, becoming Perfect Chaos, and wrecking havoc until it is locked in the Master Emerald, along with Tikal.
Much later, at the start of Sonic Adventure, Chaos is released by Dr. Eggman so that he can use Chaos to aid him in conquering the planet by collecting the Chaos Emeralds and feeding them to it, making Chaos stronger with each emerald. Once Chaos transforms into Perfect Chaos and floods Station Square but is defeated by Super Sonic after Sonic absorbs all the remaining positive energy from the Emeralds.
Chaos can transform into various forms, depending on how many emeralds have been absorbed. As a result, it appears several times during Sonic Adventure as bosses. Depending on how many emeralds he absorbs, the stronger Chaos is, starting from Chaos 0, and working his way up to Perfect Chaos.
 Outside The Games
Chaos also appeared in Sonic the Comic. In a bid to uplift Dr. Robotnik from his depression after being defeated by Sonic, Grimer unleashed Chaos with the mistaken belief that a device Robotnik created could control him. After an unsuccessful attack on Chaos, Sonic absorbed masses of Chaos Energy, turning his eyes green. Chaos proved a powerful foe, killing the freedom fighter Johnny Lightfoot and destroying parts of the Metropolis Zone. Chaos also had the ability to generate fear to shield him from attack.
Sonic went back to the time of the ancient Echidnas and saw the origin of Chaos. He was originally a fish-like creature who fought using an armoured battle suit. After Sonic removed him from the suit, a Chaos Energy flood began and his comrades left him. He merged with this and became Chaos. Returning to his own time, Sonic eventually confronted Chaos in his Perfect form. In the comics, Perfect Chaos resembled a large octopus. Before Sonic could be defeated, Super Sonic returned and removed the fish from Chaos. Recharged with Chaos Energy, Super Sonic hurled it into a pool where it was presumably eaten.
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