Which Assassin's Creed game first introduced a playable multiplayer mode?

Assassin's Creed: Brotherhood was the first game in the series to introduce a playable multiplayer mode.

Released in 2010, Brotherhood added competitive multiplayer alongside its single-player campaign. Players controlled trained Templars known as Abstergo agents and hunted other players in urban environments while trying to remain inconspicuous.

The multiplayer design emphasized stealth rather than conventional action-game firefights. Players used crowds, disguises, rooftops, and careful observation to identify targets, while targets attempted to detect and evade their pursuers.

Assassin's Creed II came before Brotherhood but did not include the series' competitive multiplayer. Revelations later expanded the multiplayer system with additional maps and characters. Brotherhood is also notable for continuing Ezio Auditore's story and for focusing on his campaign against the Borgia in Rome.