Assassin's Creed began as a proposed spin-off of which game series?

Assassin's Creed began as a proposed spin-off of the Prince of Persia series.

During development at Ubisoft Montreal, the team explored ideas for a Prince of Persia project centered on a non-player character who would protect the prince. That concept developed into a larger game focused on an Assassin, eventually becoming Assassin's Creed rather than a conventional Prince of Persia sequel or spin-off.

The connection is visible in the original game's emphasis on acrobatic movement, climbing, rooftop traversal, and historical adventure. Patrice Désilets served as creative director, and the project evolved substantially before its 2007 release.

Assassin's Creed later became its own franchise with separate characters, mythology, and gameplay systems. It is therefore inaccurate to call the released game a Prince of Persia title, even though its development history began with that proposal.