Dark Gaia

Dark Gaia (ダークガイア) is the main Antagonist of Sonic Unleashed and a gargantuan monster (referred to by Prof. Pickle as a hyper energy organism), the representation of darkness, night and destruction, who was released from the center of the Earth by Dr. Eggman's Chaos Energy Cannon 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. Dark Gaia is the main antoganist of Sonic Unleashed.

Dark Gaia's purpose is to gather energy while sleeping at the planet's core over millions of years, then destroy the world upon awakening for it to be rebuilt by Light Gaia, a process repeated since time began. Because Dark Gaia awoke prematurely when Eggman fired his beam, its pieces scattered throughout the world, and its influence affected people negatively. 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 four unaffected are Amy Rose, Professor Pickle, Dr. Eggman, 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 (nearly mimicking how Perfect Chaos shot down Eggman's Egg Carrier 2 after he began attacking Station Square in Sonic Adventure). Dark Gaia then re-absorbs its lost power from Sonic, costing him his Werehog form (which appears to injure Sonic in the process), 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 four extra arms (the in the transformation sequence it's only shown growing two 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.

Video of battle with Dark Gaia: 

Battle
How Dark Gaia fights varies between the versions of Sonic Unleashed. In the Wii & PS2 versions, when fighting the Gaia Colossus, he simply attacks with his claws while Chip dodges and counter-attacks with punches. This fight is generally considered akin to playing the Punch-Out! series of games. In its final form, Perfect Dark Gaia, he will attack by swinging his claws and laser fire from its central mouth/eye. Its eyes are again its weak points and when Sonic tries to attack them Dark Gaia, calls up a number of large molten rocks to try and slow him down.

In the Xbox 360 and PS3 versions of the game, the fight with the Gaia Colossus occurs a different way. As opposed to a simple fist fight, Chip has Gaia Colossus flying towards Dark Gaia from a distance and attack it with a quick-time event. During this time, Dark Gaia tries stop him by throwing massive molten rocks, and firing a laser from its mouth when the Gaia Colossus starts to get closer. When Sonic tries to attack Dark Gaia's eyes, he has a time limit before Dark Gaia blasts the Gaia Colossus away. As Perfect Dark Gaia, he starts the battle by using his tentacles to create a force field that not even Super Sonic can break through. The force field can only be broken when all of Dark Gaia's tentacles, which the tips of extend outside the force field. Dark Gaia can still attack through the force field and does so despite Chip attempts to divert his attention. After the force field is broken, the battle is simply turned into a button pressing mini game where Super Sonic has to land hits on each of Dark Gaia's arms when it attacks Chip and then land a hit on one of his eyes.

In both versions, Dark Gaia's attacks can deplete rings from Super Sonic.

Trivia

 * Dark Gaia is somehow similar to Encablossa from the Kingdom Under Fire series of Xbox
 * Dark Gaia also resembles the second form of the final boss in the game, Terranigma, and in fact the one in Terranigma is also called "Dark Gaia."
 * Sonic in his normal form actually inflicted a significant amount of damage on Dark Gaia before he became Perfect Dark Gaia.
 * Dark Gaia is the third boss in the series (the first being Dr. Eggman in his final machine glitch in Sonic 3 as well as his final robot in Sonic Rush and the second being Solaris) whose attacks can physically damage Super Sonic and cost him rings. Alf-Layla-wa-Layla could also cost Darkspine Sonic rings, but 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.
 * Dark Gaia and Chip could be considered as opposite brothers,as there are many differences that could support this. For one thing, Chip's real name is Light Gaia, a direct opposite to the name Dark Gaia. Also, both are "gods" created to destroy each other.
 * Dark Gaia also appears to be able to change the composition of its skin. In the cutscene when it transforms from Dark Gaia to Perfect Dark Gaia, its skin appears to be stretchy but firm, while the skin on the opposite side of its body appears to be brittle.
 * Dark Gaia could be considered the more heartless, sinister twin of Perfect Chaos as both are considered deities and they both have a perfect form. Both monsters have sent Eggman flying away by their own powers, refusing to listen to him before they are taken down by the power of Super Sonic. However, unlike Chaos losing all of his fury and rage after the fight, Dark Gaia will probably continue being a monster as it is re-sealed in the planet.
 * In addition, Dark Gaia is the second character in the series to have "Perfect" as a prefix to his final form.
 * If Dark Gaia and Solaris are both god like beings in the Sonic universe, it could be easily said that Solaris is the stronger of the two (it only took Super Sonic's power to defeat Dark Gaia where it took Sonic, Shadow AND Silver in their super states to finish Solaris, and even then the three of them could only disable him, not kill him). However, there was another god (i.e. Chip) fighting Dark Gaia with Super Sonic in the Gaia Colossus powered by all seven Chaos Emeralds, so Dark Gaia could possibly be the stronger of the two. But comparing their capabilities, Solaris may be more powerful due to the fact he could exist in all three time periods and his ability to engulf all timelines before causing time and space to collapse.
 * Dark Gaia is the second Sonic character to be named after a Greek god; the first was Chaos. This leads to an amusing situation in that Dark Gaia is referred to as a male, but was named after a goddess. This may explain why one of the sub-bosses it spawns is referred to as "Big Mother".
 * 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 earth, yet during the final fight, he is noticeably smaller and fits inside the earth with plenty of extra room. This is probably due to Gaia's mass being spread out and incomplete, as SA-55 states later on.
 * Perfect Dark Gaia is the largest final boss to date.
 * Dark Gaia is the only Boss in Sonic Unleashed with "dark" as a prefix that you fight against with normal Sonic. Also, the Egg Dragoon is the only Boss with an "Egg" as prefix you fight against with Sonic the Werehog.
 * Dark Gaia has always been bigger than Chip, even when the latter turned into Gaia Colossus.
 * Dark Gaia is the reason Sonic Unleashed is rated E10+, as he leaks green blood during his transformation into Perfect Dark Gaia. This blood is also seen in his "death" scene.
 * Other than Sonic, the only other beings Dark Gaia could not affect were Chip, Dr. Eggman, Professor Pickle, Tails and Amy. The following reasons are possible:
 * Professor Pickle: He was in possession of the Gaia Manuscripts.
 * Tails and Amy: They have a strong connection to Sonic, who himself has an indomitable will.
 * Dr. Eggman: He was in control of Dark Gaia until the end of the game
 * Chip: He is Dark Gaia's counterpart, and it is inferred that he is unaffected due to opposing powers.


 * Dark Gaia's power, as explained by Professor Pickle, is drawn from hatred and sadness manifested by the people on the planet

Video footage

 * Part 1
 * Part 2