Perfect Chaos is a main boss in Sonic Generations. It is the boss of the Dreamcast era. This boss is from Sonic Adventure.
In the original battle, it was played as Super Sonic, but it is played by Modern Sonic in this game. You boost a lot in this battle. You go though 3 sections of platforming, then you boost on the water to Perfect Chaos. You attack it's head 3 times, then you do more platforming. Then you attack its head the final time, then the battle is won.