Dark Gaia is a hyper energy organism and the physical incarnation of darkness, night and destruction and one of the main antagonists in Sonic Unleashed and the devil of Sonic's world. Since the beginning of time, Dark Gaia has fought Light Gaia in an perpetual cycle where Dark Gaia would destroy Earth after sleeping in the planet's core for millions of years for then to have Light Gaia restore the planet and put Dark Gaia back to sleep to continue the cycle.
In Sonic Unleashed, Dark Gaia was awoken prematurely by evil Dr. Eggman, who planned to use the power of the great beast to conquer the world and establish his Eggman Empire, but broke apart and scattered across the world. After being reassembled, Dark Gaia attempted to resume its mission to destroy the world, but was defeated by Sonic the Hedgehog and Light Gaia, where it then returned to its natural cycle once more.
- Being the demonic incarnation of darkness, night and destruction, Dark Gaia may exist within every planet in Sonic's universe as separate individual incarnations that perform the same duties on each planet. This could also be true for Light Gaia. --> (Dark Gaia's duty perform destroying?)
- There is debatable that Dark Gaia is a Devil or God. Maybe Dark Gaia is only a god whom looks like a devil.
- Dark Gaia is possible to be one of the worst antagonist characters in Sonic series, because Dark Gaia destroys only for destroying and seems not have self awareness.