Which game first introduced playable multiplayer to the main Assassin's Creed series?

Assassin's Creed: Brotherhood first introduced playable multiplayer to the main Assassin's Creed series.

Released in 2010, Brotherhood added competitive multiplayer alongside its single-player campaign. Its multiplayer modes focused on stalking targets, blending into crowds, and timing attacks while other players hunted the participant. This adapted the series' social-stealth ideas to player-versus-player matches.

The single-player story continues Ezio Auditore's adventures in Rome, where he works to rebuild the Assassin Brotherhood and oppose the Borgia family. The multiplayer component was separate from the main historical campaign but used the franchise's Abstergo framing device.

Assassin's Creed II had no multiplayer mode, while Revelations continued the multiplayer structure in 2011. Later entries changed or removed multiplayer in different ways. Therefore, the key distinction is that Brotherhood was the first main-series game to make multiplayer a dedicated playable feature, not merely the first title to mention online play.