Which Assassin's Creed game was the first main entry to feature online multiplayer?

Assassin's Creed: Brotherhood was the first main Assassin's Creed game to feature online multiplayer.

Released in 2010, Brotherhood added a dedicated multiplayer mode in which players controlled trained agents competing in assassination contracts. The mode emphasized blending into crowds, identifying targets, and escaping pursuers rather than relying only on direct combat. It used the series' social-stealth systems in competitive matches.

The single-player campaign continues Ezio Auditore's story and is set primarily in Rome. Ezio leads the Assassins against the Borgia family and works to rebuild the Brotherhood in Italy. The game also expands the management of recruited Assassins and introduces a large Roman environment.

Assassin's Creed II had no online multiplayer mode. Revelations and later entries continued or changed the multiplayer concept, while Assassin's Creed III also included online modes. Brotherhood's multiplayer is therefore a milestone for the main series, separate from later cooperative features such as those in Unity.