Which Assassin's Creed game introduced the franchise's first cooperative multiplayer missions?

Assassin's Creed Unity introduced the franchise's first cooperative multiplayer missions. Released in 2014, it allowed groups of up to four players to work together in specially designed missions.

The cooperative content uses the same Paris setting as the main campaign, but players control customized versions of Arno rather than separate historical protagonists. Missions can involve assassinations, thefts, and other objectives that reward coordinated movement.

Earlier Assassin's Creed games had competitive online multiplayer, especially Assassin's Creed Brotherhood and Revelations. That mode was different from Unity's cooperative format, which emphasizes teamwork against computer-controlled enemies.