Chaos is a liquid being some call 'the god of destruction',  he is the protector of the chao. The chaos feed also like chaos, on chaos energy from the chaos emeralds. In the ancient past the echidna tribe attacked and killed most of the chao for the chaos emeralds. When chaos learned of this he was enraged and killed most of the echidna race, afterwards though chaos was still angry and planned to become perfect chaos by absorbing energy from the chaos emeralds. A surviving echidna called Tikal who's life was spared because she tried to stop her father the leader of the echidnas Pochacamac from destroying the chao, learned of chaos' plan and sealed herself and chaos in the master emerald. Centuries later chaos was freed by Dr. Robotnik and the doc persuaded him to help him take over mobius. Chaos absorbed power from all the chaos emeralds to become perfect chaos and destroyed station square. Tikal and Tails explained that chaos only absorbed the negative power from the chaos emeralds so sonic absorbed the positive chaos energy to become super sonic who defeated chaos and restored harmony.