To understand why the is a fantasy, you must look at the hardware specs:
The series began as a prototype for a new Prince of Persia game titled PoP: Assassins . Because the Prince of Persia trilogy was a staple of the PS2 library, many players expected the successor to appear there as well.
The first Assassins Creed game was released in 2007 for the PlayStation 3, Xbox 360, and Microsoft Windows. However, due to the PS2's popularity and relatively low system requirements, Ubisoft, the game's developer, decided to create a version of the game for the PS2. Assassins Creed: Brotherhood, the second game in the series, was initially intended to be released on the PS2, but ultimately, it was canceled.
The PS2 simply does not have enough memory to load the crowd AI (which was revolutionary in 2007) or the draw distance of the Kingdom. Even the PS3 and Xbox 360 versions struggled with frame rates during the famous "flag collection" sections.
Instead of chasing a nonexistent PS2 version, play the real PC version or use emulators for supported platforms:
This article will dissect the confusion surrounding the search query, explain why you won't find a legitimate ISO of the first game for the PS2, and guide you toward the actual Assassin’s Creed titles available on the platform.