The ancient evil central to Sonic Adventure is Chaos.
In the 1998 Dreamcast game Sonic Adventure, Doctor Robotnik discovers Chaos, an ancient creature connected to the Chao and the Master Emerald. Chaos once protected the Chao, but became enraged when an echidna tribe tried to seize the power of the Chaos Emeralds. Empowered by the Emeralds, it transformed into Perfect Chaos and destroyed the tribe.
Tikal, a young echidna who opposed the tribe's violence and befriended Chaos, sealed both herself and the creature inside the Master Emerald. Centuries later, Robotnik shatters the Emerald to release Chaos and uses the Chaos Emeralds to increase its power. Sonic and the other playable heroes then work to stop the threat.
The name can cause confusion because Chaos is not one of Robotnik's robots or a later rival character. Mephiles belongs to Sonic the Hedgehog (2006), while Infinite is associated with Sonic Forces and Zavok with the Deadly Six. In Sonic Adventure, Chaos is the central supernatural antagonist.