What was Assassin's Creed originally conceived as a spin-off of?

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

The project began as a proposed Prince of Persia spin-off known as Prince of Persia: Assassin. Its design explored a secondary Assassin character and expanded the parkour and historical-adventure ideas associated with the Prince of Persia series.

As development continued, the concept grew far enough from its source material to become a separate intellectual property. Ubisoft then developed it into Assassin's Creed, with Altaïr Ibn-La'Ahad as the protagonist and the Third Crusade as the historical setting.

The connection explains why early discussions of the game often mention Prince of Persia. Assassin's Creed is not a direct Prince of Persia sequel, however, and its modern-day Animus framework, Assassin Brotherhood, and Templar conflict became its own defining mythology.