DLC Sonic Adventure: Sonic Adventure DX Upgrade (2010)

Doctor Robotnik seeks a new way to defeat his longtime nemesis Sonic and conquer the world. During his research, he learns about an entity called Chaos—a creature that, thousands of years ago, helped to protect the Chao and the all-powerful Master Emerald, which balances the power of the seven Chaos Emeralds. When a tribe of echidnas sought to steal the power of the Emeralds, breaking the harmony they had with the Chao, Chaos retaliated by using the Emeralds' power to transform into a monstrous beast, Perfect Chaos, and wipe them out. Before it could destroy the world, Tikal, a young echidna who befriended Chaos, imprisoned it in the Master Emerald along with herself. Eggman releases Chaos and Sonic and his friends must act against Eggman's plans and prevent the monster from becoming more powerful.