Which Assassin's Creed game introduced the Animus Database feature known as the Codex?

Assassin's Creed II introduced the Animus Database feature known as the Codex.

In the game, Ezio Auditore collects pages of a Codex written by Altaïr Ibn-La'Ahad. Leonardo da Vinci translates and studies the pages, gradually revealing information about the Apple of Eden, the Assassin Brotherhood, and the wider conflict between Assassins and Templars.

The Codex is not merely a menu of optional lore: assembling its pages is tied to the story and eventually helps reveal a route to the game's final location. The pages also connect Ezio's Renaissance adventure to Altaïr's medieval storyline.

Later games expanded their encyclopedic databases with entries about characters, places, and historical subjects. That broader reference system can make the Codex easy to confuse with a generic database, but in Assassin's Creed II the term specifically points to Altaïr's collection of writings and the information it contains.