||And the world shall be plunged into the dark of night, the dark of destruction.
— The Gaia Manuscripts
Perfect Dark Gaia is Dark Gaia's final form and the last boss of Sonic Unleashed. A hyper-energy organism, millions of years old, it has fractured the world and destroyed civilization untold numbers of times before, and now, thanks to Eggman, attempts to do so again. Dark Gaia powers up to this form after an inconclusive first battle against Sonic and the Gaia Colossus in the world's core by absorbing the last bit of its power that resided in Sonic, giving him his Werehog form. Sonic then uses the Chaos Emeralds to fight him as Super Sonic, teaming up once again with Chip in the Gaia Colossus.
Boss Fight - 360/PS3 version
In the 360/PS3 version, the fight starts with Super Sonic flying through the lava chamber towards the boss, before being blocked by its hemispherical shield. The Gaia Colossus breaks through to distract Perfect Dark Gaia, while Sonic is directed to deal with the sinuous snake-heads that sustain the barrier. To do this, Super Sonic has to skate across the shield's surface, Sonic Boosting into the heads before they retract back inside. This section requires you to dodge huge rock chunks already embedded in the shield, as well as meteors and lasers hurled by Perfect Dark Gaia. The Gaia Colossus' health bar serves as a time limit to defeat the snake-heads (if Gaia Colossus dies, you will lose a life).
Once you destroy the shield, you will go through Quick-Time Events of Super Sonic blazing through Perfect Dark Gaia's limbs to protect Gaia Colossus, and the Gaia Colossus blocking an energy beam from Perfect Dark Gaia. The finale of the boss will have Super Sonic crashing into Perfect Dark Gaia's eyes.
Boss Fight - Wii/PS2 version
In the Wii/PS2 version, the player must use a Super Sonic Boost and attack each of Perfect Dark Gaia's seven eyes while dodging meteors and claw attacks.