Sonic Unleashed 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 Unleashed. The Werehog gameplay was conceived to help introduce newer gamers unfamiliar with the Sonic franchise to the series. 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 the Xbox 360 and PlayStation 3, one by Sonic Team and Dimps for the Wii and PlayStation 2, and one by Gameloft for mobile phones. The game was release 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, critical reception was mixed. Reviewers praised certain elements, such as the sense of speed in daylight stages and the graphics and audio that make up the environments, but criticized others, such as the Werehog game mechanic, as well as several gameplay and design concepts. Many felt Unleashed was not the game to reinvigorate the series. Sonic Unleashed was delisted from retailers in 2010, following Sega's decision to remove all Sonic games with below-average Metacritic scores from sale. Despite this, the game was released on the PlayStation Network in April 2014 and on PlayStation Now in March 2011 and was also made available on the Xbox One via backwards compatibility on November 29, 2018. The game also remains a fan-favorite amongst fans of Sonic games and the platforming genre.