Sonic Unleashed

Sonic Unleashed is a platform game in which the player controls the titular Sonic the Hedgehog in two modes: fast-paced levels that take place during daytime, showcasing and using Sonic's trademark speed as seen in previous games in the series, and slower, night-time levels, during which Sonic transforms into the Werehog, and gameplay switches to an action-based, brawler style of play, in which Sonic battles Gaia enemies (those created by the main enemy in the game, Dark Gaia). Each level takes place on a particular continent, each of which is based on a real-world location. In sections of the PlayStation 3 and Xbox 360 versions of the game, the player may choose to advance the time of day in order to play as either Sonic or the Werehog; in the PlayStation 2 and Wii versions, time is advanced automatically.

Daytime levels focus on Sonic's speed, and to this extent, sees the player control Sonic through fast-moving stages containing both 2D and 3D styles of gameplay. 2D sections are reminiscent of the Mega Drive/Genesis-era Sonic games where the player controls Sonic in a side-scrolling fashion. The game also features a gameplay mechanic previously used in the Sonic Rush series called the Sonic Boost (or just "Boost"), which greatly increases Sonic's speed, allowing him to smash through objects, destroy enemies instantly, or even access different level paths. In the PlayStation 3 and Xbox 360 versions, an on-screen "Ring Energy" meter displays how much Boost is available.

Sonic is pursuing his nemesis, Doctor Eggman, bounding around a fleet of spaceships. Sonic transforms into Super Sonic and corners Eggman on a nearby space station. However, it is revealed to be a powerful new weapon known as the Chaos Energy Cannon, which not only traps and forcefully reverts Sonic to his normal state and extracts the Chaos Emeralds from him, but also drains them of their power, resulting in them becoming inert. Eggman then uses the Emeralds' energy to fire an enormous laser that unleashes an ancient evil known as Dark Gaia from the planet's core, which shatters its surface into seven pieces.

In addition, the process has the unforeseen side-effect of transforming Sonic into a "Werehog"—a werewolf-like affliction that strips the hedgehog of his speed but bestows greater strength and abilities—during nighttime. Eggman ejects Sonic into space, who then lands safely onto the planet below.

Development on Sonic Unleashed began in mid-2005 at Sonic Team, with Yoshihisa Hashimoto as director. It was originally intended to be the third installment of the Sonic Adventure series and subsequently, at an early development stage, had the working title Sonic World Adventure, complete with a work-in-progress logo. However, the development team began to introduce enough new innovations to separate it from the Sonic Adventure games, and so a new title, Sonic Unleashed, was decided.

Sonic Team decided early on in the development process to reduce the number of characters present in the game, as well as to make Sonic the only playable character; this decision was taken to get more quality out of fewer characters. While it felt that these ideas combined with traditional Sonic gameplay was a good beginning, it also had to consider how to introduce newer gamers unfamiliar with the Sonic franchise to the series, and so the concept of the Werehog was born, in addition to exploring its own mythology and ideas for the story. Director Yoshihisa Hashimoto felt from the outset that there would be both praise and criticism, but hoped that long-term Sonic fans would understand and empathize with the ideas and direction the team had taken. The game's human non-player characters were designed by Japanese artist team Gurihiru. The opening was animated by Marza Animation Planet with Shinji Aramaki as director.

Initial anticipation when the first media for Unleashed was revealed was high, as the demonstration videos hinted at a possible return of Sonic to his traditional platforming roots, especially because of the series' declining quality in recent years, and a number of poorly received games in the franchise that preceded it, such as the 2006 game Sonic the Hedgehog.

Sonic Unleashed received "mixed or average" reviews, with Metacritic aggregate scores of 60 and 54 out of 100 for the Xbox 360 and PlayStation 3 versions respectively, and 66 out of 100 for the Wii and PlayStation 2 versions. The added element of motion controls for the Werehog sections, as well as text-based hub worlds and better Werehog level design and camera system, were reasons cited for the higher review scores for the Wii and PlayStation 2 versions of the game, though a few review websites, such as 1UP, gave the Wii version a lower score than its Xbox 360 and PlayStation 3 counterparts. Nevertheless, the game was a commercial success and sold 2.45 million units combined making it Sega's third bestselling game during their last fiscal year period of 2008.

Speaking after the game's release, Sonic Team member and Sonic and the Black Knight (2009) producer and director Tetsu Katano remarked that although he did not feel the Werehog concept was a mistake, time and resources were a limiting factor in the game's production. He also remarked that the Werehog may reappear in future games, or possibly in a sequel to Sonic Unleashed, should one be made. In the lead up to the release of Sonic Colors (2010), producer Takashi Iizuka expressed regret at the inclusion of the Werehog, stating that it clashed with Sonic's core gameplay. Another short featuring the ghost characters from Night of the Werehog, titled Ghost Tale, was released by Marza Animation Planet on October 31, 2023, 15 years after Night of the Werehog's release.

The gameplay of the daytime stages would be reused and expanded upon to become the defining gameplay of the franchise for the following decade, with Sonic Colors, Sonic Generations (2011), and Sonic Forces (2017) taking inspiration from Unleashed. The home console/PC versions of Sonic Generations contain remakes of the "Rooftop Run" stage and the Egg Dragoon boss fight. Sonic Forces also contains a re-imagined version of the Egg Dragoon boss. The Hedgehog Engine would be refined for future entries, being reworked into Hedgehog Engine 2 starting with Forces.