Dark Gaia (ダーク・ガイア Dāku Gaia) is the main antagonist of Sonic Unleashed. A gargantuan monster (referred to by Professor Pickle as a hyper energy organism), the representation of darkness, night and destruction, who was released from the center of the planet by Dr. Eggman's Chaos Energy Cannon at the start of Sonic Unleashed.
History
Dark Gaia awakes every 7000 years and is out back to sleep by Light Gaia, and the cycle repeats.
Sonic Unleashed
Eggman planned to use the power of Dark Gaia in order to conquer the world and establish his Eggman Empire. It is what spawns the monsters that plague the world during the night stages of the game. During the final battle between Super Sonic, Chip, and Dark Gaia, Sonic and Chip were able to defeat it , in turn, putting it back to sleep and stopping the darkness from causing the end of the planet. Before the start of the battle with Dark Gaia after it refused to listen to eggmans orders, Dark Gaia took back its remaining darkness that caused Sonic to turn into the Sonic the Werehog at night.