Which Assassin's Creed game first introduced competitive online multiplayer?

Assassin's Creed: Brotherhood first introduced competitive online multiplayer to the series.

Released in 2010, Brotherhood included a multiplayer mode in which players controlled trained Templar agents hunting one another in crowded historical environments. The mode emphasized stealth, disguise, careful movement, and timing rather than conventional running-and-gunning combat. Players earned points for identifying and assassinating assigned targets while avoiding their own pursuers.

The multiplayer characters included archetypes such as the Doctor, the Courtesan, the Executioner, and the Harlequin. Maps and modes were expanded in later releases, especially Assassin's Creed: Revelations and Assassin's Creed III, but those games did not originate the feature.

This fact is sometimes confused with Assassin's Creed II because Brotherhood continues Ezio's story and uses the same Renaissance-era continuity. Assassin's Creed II was a single-player game. Brotherhood's online component was separate from its main campaign, which is set primarily in Rome and follows Ezio's struggle against the Borgia regime.