Which Assassin's Creed game was the first in the series to feature cooperative multiplayer missions?

Assassin's Creed Unity was the first Assassin's Creed game to feature cooperative multiplayer missions.

Released in 2014, Unity added cooperative missions for up to four players. These missions are separate from the main single-player campaign and allow players to work together using the game's parkour, stealth, combat, and assassination systems in revolutionary Paris.

Earlier games had multiplayer features, but they were different in structure. Assassin's Creed Brotherhood and Assassin's Creed Revelations included competitive online multiplayer modes in which players hunted one another. Those modes did not provide the same cooperative mission format introduced by Unity.

Assassin's Creed Syndicate followed Unity and retained a single-player focus rather than continuing its cooperative campaign structure. This distinction matters because “multiplayer” can refer broadly to any online mode, while the question specifically concerns cooperative missions in the main game framework.