Dark Gaia is the main antagonist and final boss of Sonic Unleashed. He is a gigantic creature who was released from the core of the planet by Dr. Eggman. Sonic and Gaia Colossus, which is Chip's new form, are used to tackle this boss battle.
Dr. Eggman used the Chaos Emeralds he obtained to power up his device and launch an enormous laser into the Earth. It led to a split of the world into seven large chunks. It also led to Dark Gaia awakening from the Earth's core. Moments later, it's shown that Dark Gaia couldn't sustain itself, so the creature broke apart into countless pieces of dark energy, part of which Sonic accidentally absorbed, creating his Werehog form. Many enemies were formed from Dark Gaia's energy fragments scattered throughout the Earth and were mainly causing havoc during the evening.
Throughout the game, Dr. Eggman was working on a device to help Dark Gaia regain all of its energy, while at the same time using some of the creature's power to create Eggmanland, as well as his machine, Egg Dragoon. Sonic and Chip on the other hand, tried to put the planet together before Dark Gaia fully awakens. After confronting Eggman, in his Egg Dragoon machine, Dark Gaia ross and took the remaining dark energy from Sonic's body, allowing the creature to fully complete itself.
Suddenly Chip, who's known as Light Gaia, summoned the seven Gaia Temples to create an enormous stone body called the Gaia Colossus. He and Sonic, who's inside the Gaia Colossus, both prepare to take on Dark Gaia.
 Boss Battle
The PS3 and Xbox360 versions of this boss battle plays out differently than the Wii and PS2 versions. Both versions are split into three phases.
 Playstation 3 / Xbox 360 Versions
The battle begins with the player controlling Gaia Colossus, who's at a fair distance away from Dark Gaia. Gaia Colossus must make his way to Dark Gaia, while the creature launches attacks, such as large fire balls and a laser beam. Gaia Colossus has poor mobility, therefore the best thing to do is to punch the fire balls when they approach you, and shield yourself when he launches the laser, as both attacks will drain your health if you take a direct hit. When Gaia Colossus reaches Dark Gaia, you must attack him by using QTE's(quick time events), which if done correctly, Gaia Colossus will punch the creature, dealing damage.
Afterwards, Dark Gaia will grab Gaia Colossus' body and players will take control of Sonic, who will have to run through Gaia Colossus' body to reach Dark Gaia, which is the second phase. As he's running, many energy beams will be tossed his way and Sonic must also avoid the purple tentacles. The first time, Sonic will only have about 15 seconds to reach the end, where players must perform the QTE, which if done correctly, will lead to Sonic Homing Attack the first eye.
Following that, Dark Gaia will knock them both back a fair distance, and the entire process of the battle will repeat two more times, in which Gaia Colossus must proceed towards Dark Gaia again, then Sonic will run through Gaia Colossus to attack Dark Gaia's two remaining eyes. The second time, Sonic runs through Gaia Colossus' body, he will have about 20 seconds to get to the second eye, and about 35 seconds in his final run to reach the third and final eye.
 Wii / Playstation 2 Versions
The first phase of this boss fight in the Wii / Playstation 2 versions plays out completely different from the Playstation 3 / Xbox 360 versions. Players take control of Gaia Colossus and he is already close to Dark Gaia. You must punch Dark Gaia at the right time while avoiding the claws that the creature will attempt to hit you with. Failure to dodge the claws at the correct time will lead to Gaia Colossus taking damage. This part is played out similarly to the style of the Punch-Out games. The first phase finishes when you drain all of the creature's health.
There are some differences with the second phase. When running through Gaia Colossus with Sonic in this version, there are no time limits. Also, instead of alternating 3 times between Gaia Colossus and Sonic's running section on its body, Sonic must run through three consecutive sections, designed exactly as the Playstation 3 / Xbox 360 versions, and attack all 3 eyes.
After attacking the creature's 3 eyes in both versions, it will lead to the third and final phase, in which Dark Gaia completes his transformation and becomes Perfect Dark Gaia.
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