Dark Gaia

Dark Gaia is a titanic creature and a corrupted elemental, and one of the main antagonists of Sonic Unleashed.

Dark Gaia was the "dark" half of the planet while Chip (also known as Light Gaia) represented the lighter half of existence. Every millions of years, Dark Gaia would emerge to destroy the world, but Chip would always put it back together and seal Dark Gaia in the earth's core again.

History
When Dr. Eggman used the Eclipse Cannon to split Earth into pieces, Dark Gaia was awoken and began to destroy the natural order while Chip was born into the mortal world as a small fairy-like creature. It is what spawns the monsters that plaque the world. However Dark Gaia was awakened prematurely so he was split into pieces, which Eggman eventually brought together.

Upon becoming mostly whole and emerging from the lava at the center of the earth, Dark Gaia ignored Eggman's orders and sent him flying into the distance. Chip fought Dark Gaia, and eventually Sonic fought him as well, before Dark Gaia absorbed the power within Sonic that made him a werehog, becoming Perfect Dark Gaia. Sonic used the Chaos Emeralds to turn into Super Sonic and defeated him, smashing through his center eye. Slowly decaying and gushing green blood, Dark Gaia fell back into the lava. Though the world was saved, Professor Pickles explains that light cannot exist without darkness and thus it's possible Dark Gaia is still alive as part of the planet's cycle.

Trivia

 * Dark Gaia is the biggest of Sonic's enemies to be fought.
 * Dark Gaia resembles the second form of the final boss in the game Terranigma. In fact, the final boss in Terranigma is also called "Dark Gaia."
 * In the Wii/PS2 version, when Sonic must attack Dark Gaia's eyes by going across the Gaia Colossus, it is the only time Sonic grinds in 3D, as in this version all rails are strictly 2D only. It is also the only time when Sonic must use the quick step on a grind rail.
 * His final battle is different in the Wii/PS2 version and the 360/PS3 version. In the Wii/PS2 version Sonic must collect rings and smash through each of his eyes one by one, and then do the same with his center eye to finish him off. In the 360/PS3 version, Sonic must take out his shield by killing the snakes projecting the shield while Chip fights Dark Gaia inside. Then Dark Gaia grabs onto Chip, and the player must press buttons as they appear to make Sonic attack Dark Gaia's hands. Then Dark Gaia shoots an energy wave at Chip, which the player must tap a button repeatedly to block. After Chip punches Dark Gaia, the player must press a final button to make Sonic smash through Dark Gaia's center eye.
 * The monsters that are made of Dark Gaia's essence somewhat resemble the Heartless from the Kingdom Hearts series of video games.
 * The name Gaia is derived from an ancient Greek goddess, who was the personification of the Earth. (A previous Sonic villain, Chaos, was also named after a Greek god.) Although named after a goddess, Dark Gaia is referred to as male.
 * Dark Gaia seems to change size throughout the course of Sonic Unleashed. When he is first awakened, he is large enough to put one hand on either side of the planet, yet during the final fight, he is noticeably smaller and fits inside the planet with plenty of extra space in the planet's core. This is probably due to Dark Gaia's mass being spread out and incomplete, as Orbot states later on.
 * Dark Gaia might have been the reason that Sonic Unleashed was given the ESRB rating of E10+, as he leaks green blood during his transformation into Perfect Dark Gaia. This blood is also seen in his "death" scene.
 * In Sonic Adventure 2, Tikal's earthquake attack was called "Wrath of Gaia". This is most likely a coincidence.
 * Dark Gaia could arguably be considered neutral since destroying the world is it's natural role.
 * All of the bosses in the game with a Dark prefix appear to elemental:
 * Dark Gaia Phoenix - Fire
 * Dark Moray - Ice
 * Dark Guardian - Thunder
 * Dark Gaia - Earth
 * Dark Gaia appears to have a resemblance to Iblis. Dark Gaia has Shadowy dark minions like Iblis has Fire minions, and they both have a desire to destroy the world.
 * Considering Dark Gaia needs a certain amount of power to become Perfect Dark Gaia,he might have never transformed if he didn't absorb the Werehog energy from Sonic.
 * Dark Gaia is one of the three final bosses to have an orchestrated version of the game's main theme as the final boss music (specifically Endless Possibility),the other two being Solaris Phase 2 (His World) and Nega Wisp Armor Phase 2 (Reach For The Stars).
 * The possibility that Dark Gaia is still alive despite his apparent death is similar to theories that Lord Darkar from Winx Club might have survived; the balance between light and darkness. However, in Darkar's case, he was explicitly stated to have died by Faragonda, and it is possible that his power or essence survived his death in a disembodied form (like the Great Dragon) to maintain the balance, so it's also possible this happened with Dark Gaia.