Which Assassin's Creed game introduced the series' first competitive multiplayer mode?

Assassin's Creed Brotherhood introduced the series' first competitive multiplayer mode.

Released in 2010, Brotherhood added online competitive multiplayer alongside its single-player story. Players controlled Templar agents who hunted assigned targets while trying to avoid being detected and killed by other players.

The multiplayer design adapted the series' social-stealth ideas to human opponents. Disguises, crowds, awareness indicators, careful movement, and timed abilities were more important than simply defeating enemies in open combat. Several modes and maps expanded the concept in later releases.

Assassin's Creed II had no competitive online multiplayer mode, although it established much of the gameplay and narrative framework that Brotherhood continued. Later games, including Revelations, III, and IV: Black Flag, also included multiplayer features, but Brotherhood is credited with introducing the series' competitive multiplayer experience.