Which Assassin's Creed game was the first in the series to feature playable multiplayer modes?

Assassin's Creed Brotherhood was the first Assassin's Creed game to feature playable multiplayer modes.

Released in 2010, Brotherhood added online multiplayer alongside its single-player campaign. Its multiplayer modes placed players in the roles of Templar agents who hunted one another in crowded urban environments. Players had to identify targets, use disguises, remain inconspicuous, and perform stealth assassinations rather than simply defeat opponents in open combat.

The mode included multiple characters with distinct appearances and animations, as well as scoring systems that rewarded careful pursuit and stealth. This was a major expansion of the series' social-stealth concept and preceded the multiplayer offerings in later entries.

Assassin's Creed II had no playable online multiplayer, despite introducing several systems that were later developed across the franchise. Revelations and Assassin's Creed III also included multiplayer, but both came after Brotherhood's 2010 debut.