Which Assassin's Creed game first included an online multiplayer mode?

Assassin's Creed: Brotherhood was the first Assassin's Creed game to include an online multiplayer mode.

Ubisoft released Brotherhood in 2010 as a direct continuation of Assassin's Creed II. Its multiplayer component placed players in competitive stealth matches, where they hunted assigned targets while trying to avoid becoming targets themselves. Several modes and character types supported the cat-and-mouse design.

The single-player campaign follows Ezio Auditore in Rome, where he works to rebuild the Assassin Brotherhood and oppose the Borgia family. It also continues the modern-day storyline of Desmond Miles.

Assassin's Creed II had no online multiplayer mode, although later entries expanded the feature. Assassin's Creed Revelations, released in 2011, continued the multiplayer system rather than introducing it. The online component was eventually discontinued, but Brotherhood remains the series entry credited with first bringing competitive multiplayer to the main franchise.