Which Assassin's Creed game was the first to introduce the series' multiplayer mode?

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

Released in 2010, Brotherhood added online modes in which players hunted one another through crowded city environments. Participants received targets, used disguises and stealth, and tried to identify and assassinate opponents while avoiding being killed themselves.

The single-player campaign continues Ezio Auditore's story in Rome and focuses on rebuilding the Assassin Brotherhood. Multiplayer was a separate feature rather than a replacement for that campaign, and it helped establish social stealth as a competitive format.

Assassin's Creed II preceded Brotherhood but did not include this multiplayer system. Revelations and Assassin's Creed III expanded the feature later, so they are not the first entries to introduce it.