Dark Gaia

Dark Gaia is a monster (referred to by Prof. Pickle as a hyper energy organism) who was released from the center of the Earth by Dr. Eggman's Chaos Emerald/Super Sonic-powered ray at the start of Sonic Unleashed. Eggman plans to use the power of the great beast 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.

The representation of night, darkness, and destruction; Dark Gaia awoke prematurely when Eggman fired his beam. Dark Gaia's purpose is to gather energy while sleeping at the planet's core over thousands of years, then destroy the world upon awakening for it to be rebuilt by Light Gaia, a process repeated since time began. Its pieces scattered throughout the world, and its influence affected people. The only exceptions were Sonic the Hedgehog because his will was too strong, even in his Werehog form and Light Gaia because he is Dark Gaia's opposite. The other three unaffected are Amy Rose, Professor Pickle and Tails for unknown reasons.

Over the course of the game, Dr. Eggman devises a way to gather the pieces of Dark Gaia and reassemble them, giving him the power needed to finally build Eggman Land. After Sonic (in his Werehog form) defeats Dr. Eggman's Egg Dragoon, the reassembled Dark Gaia appears. Eggman orders it to attack Sonic, but Dark Gaia, not intending to follow his orders, swats him away. Dark Gaia then reabsorbs its lost power from Sonic, costing him his Werehog form, and completes itself, gaining three extra eyes. Chip battles Dark Gaia in a giant stone robot formed by the Gaia temples known as the Gaia Colossus while Sonic attacks his eyes. While they are doing this, Dark Gaia completely regains his lost power and matures, growing seven eyes, and six arms, becoming Perfect Dark Gaia. In the end, Super Sonic and Light Gaia defeat Dark Gaia's final form, and both Light and Dark Gaia are resealed within the still-regenerating planet. Here is a video to show how to beat him: http://news.gamewinners.com/index.php/weblog/news/sonic_unleashed_final_boss_battle/

Trivia
Dark Gaia is the third boss in the game series (the first being Dr Eggman in his final machiine in Sonic 3 and the second being Solaris) whose attacks can physically damage Super Sonic and cost him rings. Dark Gaia is known to be as powerful as Solaris. Other than Solaris and Dark Gaia, Eggman and Eggman Nega's final mecha in the Extra zone in Sonic Rush can cost Super Sonic rings. Also, Alf-Layla-wa-Layla could cost Darkspine Sonic rings, but again, Sonic and the Secret Rings is a spin-off, and Darkspine was an incomplete form if you take into fact only three of the seven World Rings were absorbed.