Which Assassin's Creed game introduced playable multiplayer as a major feature?

Assassin's Creed Brotherhood introduced playable multiplayer as a major feature in the series.

Released in 2010, Brotherhood included a competitive multiplayer mode in which players hunted one another through crowded historical environments. Participants received a target while also being pursued, creating a cycle of stealth, observation, disguise, and careful timing.

The multiplayer characters used abilities and appearances based on the game's Renaissance setting. Maps included versions of locations such as Rome and Castel Gandolfo. The mode was separate from the main single-player story, which follows Ezio Auditore as he challenges the Borgia power structure in Italy.

Earlier Assassin's Creed games were primarily single-player experiences. Later entries, including Revelations, III, Black Flag, and Unity, also included multiplayer or cooperative features in various forms. Unity's cooperative missions are different from Brotherhood's competitive player-versus-player system, which is why Brotherhood is the key answer to this question.