Sonic Unleashed[b] is a 2008 platform video game in the Sonic the Hedgehog series. The story follows Sonic as he attempts to restore the world after his nemesis Doctor Eggman shatters it with a powerful laser to unleash Dark Gaia, an ancient evil, while dealing with having become a "Werehog", a werewolf-like transformation, which he gains after coming into contact with the energy of Dark Gaia. Gameplay features two distinct styles, with each being played either during daytime or nighttime. Daytime stages incorporate Sonic's traditional platforming and trademark speed, with a combination of behind-the-back third-person viewpoints and 2D side-scroller platforming; gameplay seamlessly transitions between these two views. Night-time levels see Sonic transform into the Werehog; gameplay slows down to accommodate greater platform play, and involves combat against waves of enemies using the Werehog's brute strength.
The game's development began in 2006 and lasted 18 months, after the creation of its game engine, the Hedgehog Engine. It was initially conceived as a sequel to Sonic Adventure 2 (2001), but developer Sonic Team began to introduce enough new innovations that separated it from previous games, and it was renamed Sonic World Adventure in domestic markets. The Werehog gameplay was conceived to help introduce newer gamers unfamiliar with the Sonic franchise to the series and is what influenced the use of Unleashed as a subtitle for western markets. The game's existence was first brought to light when Sega trademarked the Unleashed name in March 2008, and shortly after, images and a gameplay video were leaked. Three versions of the game were developed: one by Sonic Team for high definition consoles such as the Xbox 360 and PlayStation 3, one by Sonic Team and Dimps for standard definition consoles such as the Wii and PlayStation 2, and one by Gameloft for mobile phones. The game was released worldwide in 2008.
Public anticipation for Sonic Unleashed was high, as video game journalists saw it as a possible return to Sonic's platforming roots. While it was commercially successful, selling 2.45 million units, initial critical