Which Assassin's Creed game first introduced four-player cooperative missions?

Assassin's Creed Unity first introduced four-player cooperative missions in the main Assassin's Creed series.

Unity allows up to four players to work together online in selected missions and heists. Each participant controls a customized version of Arno Dorian, and the group can coordinate stealth, combat, and movement across the game's detailed recreation of Paris.

The cooperative missions are separate from the single-player campaign, which follows Arno during the French Revolution. Players can also explore Paris together in free roam and take on activities designed for coordinated play. This system was a major multiplayer expansion compared with earlier titles.

The feature is sometimes confused with the competitive multiplayer mode introduced in Assassin's Creed: Brotherhood. Brotherhood let players hunt one another in competitive matches, whereas Unity's headline innovation was cooperative play against shared objectives. Assassin's Creed Syndicate later included a second playable protagonist but did not introduce the same four-player cooperative campaign structure.