Which Assassin's Creed game was the first to include a competitive multiplayer mode?

Assassin's Creed Brotherhood was the first Assassin's Creed game to include a competitive multiplayer mode.

Released in 2010, Brotherhood added online multiplayer alongside its single-player campaign. The multiplayer experience placed players in crowded urban environments and asked them to identify and assassinate assigned targets while avoiding detection.

Players controlled specially designed Templar agents and used disguises, blending, stealth, and careful observation. Popular modes included Wanted and Manhunt, which gave opposing players different hunting objectives.

The game's single-player story continues Ezio Auditore's campaign in Rome, where he works to weaken the Borgia family and rebuild the Assassin Brotherhood. Because Revelations and later games also included multiplayer, they are sometimes remembered as the origin of the feature, but Brotherhood introduced competitive online play to the franchise.