Chaos (Kaosu) is a fictional character in the Sonic the Hedgehog series and the main antagonist in the video game Sonic Adventure for the Sega Dreamcast. Chaos is an immortal, ageless god-like creature that seems to be composed entirely of water (ironically, Sonic has a weakness of water), but is also said to be concentrated Chaos Emerald energy (in Sonic X, Doctor Eggman implies that it is composed of "Chaos energy in a liquid state"); it is really a mutant Chao. It was trapped inside the Master Emerald for thousands of years before being released by Dr. Eggman in a bid to conquer the world. It was also the original guardian of the Chaos Emeralds, Chao, and the Master Emerald. In Sonic Adventure, it absorbed the negative energy of the Chaos Emeralds to attain its final and strongest form.