Which 2010 Assassin's Creed game introduced the multiplayer mode to the main series?

Assassin's Creed Brotherhood introduced multiplayer to the main Assassin's Creed series in 2010. Its competitive mode placed players in the roles of Templar agents who hunted one another in crowded historical environments.

The multiplayer system emphasized blending into crowds, identifying targets, and using stealth rather than relying only on direct combat. Players received a target while another player hunted them, creating a distinctive cat-and-mouse structure.

Brotherhood's single-player campaign continued Ezio Auditore's story in Rome. The game combined the city-focused campaign with the multiplayer mode, making it the first principal series entry to offer this type of online competitive experience.

Assassin's Creed II did not include the main series' competitive multiplayer mode, and Revelations expanded the feature after Brotherhood. Later releases changed or omitted multiplayer, so the exact point of introduction is specifically associated with Brotherhood.