* The true ending: You get to witness the heartwarming conclusion of the story. This includes the rebuilding of the world and the happy reunion of Sonic and his friends.
* The final boss battle: You get to fight a powerful enemy, possibly even Dark Gaia itself, depending on which version of Sonic Unleashed you are playing.
* New gameplay modes: Some versions of Sonic Unleashed offer additional gameplay modes after defeating Dark Gaia Phoenix, such as challenge levels, time trials, or additional stages.
* Unlockable content: You may gain access to additional content like concept art, music tracks, or bonus features.
To be specific:
* If you're playing the standard version of Sonic Unleashed, you will face Dark Gaia itself after defeating the Dark Gaia Phoenix.
* If you're playing the "Ultimate Edition", you may have access to new levels and gameplay modes.
Regardless of the specifics, defeating Dark Gaia Phoenix marks the completion of the game's primary storyline and unlocks a satisfying ending and additional content.