Which earlier video-game series was Assassin's Creed originally planned as a spin-off of?

Assassin's Creed was originally planned as a spin-off of Prince of Persia.

During development at Ubisoft Montreal, the team worked on a concept connected to Prince of Persia: The Sands of Time. The idea focused on a young character serving an elite Assassin figure, but the concept expanded beyond the boundaries of a direct spin-off.

As development continued, Ubisoft shaped the project into a new intellectual property. The resulting game retained recognizable interests from the earlier series, including historical settings, acrobatic movement, stealth, and large architectural environments.

The finished Assassin's Creed introduced its own mythology, characters, and conflict between Assassins and Templars. This origin is sometimes confused with the series' later use of historical fiction or with the 2006 announcement of Assassin's Creed. The spin-off connection describes the project's development history, not the final game's ownership or story continuity.