Which Assassin's Creed game introduced playable online multiplayer?

Assassin's Creed Brotherhood introduced playable online multiplayer to the main Assassin's Creed series.

Released in 2010, Brotherhood added competitive multiplayer modes in which players hunted assigned targets while trying to avoid detection by other players. The multiplayer characters were drawn from the game's Assassin and Templar world, giving the mode a distinctive social-stealth design rather than a conventional team shooter format.

The single-player campaign continues Ezio Auditore's story in Renaissance Rome. Ezio works to dismantle the Borgia family's power and rebuild the Assassin Brotherhood, while the modern-day storyline continues through Desmond Miles.

Assassin's Creed II had online features associated with some releases and services, but it did not provide the series' first playable online multiplayer experience. Later titles, including Revelations and III, expanded or changed the multiplayer systems, which can cause confusion about where the feature began.