Which Assassin's Creed game introduced multiplayer as a major feature of the main series?

Assassin's Creed Brotherhood introduced multiplayer as a major feature of the main Assassin's Creed series.

Released in 2010, Brotherhood added a competitive multiplayer mode in which players controlled Templar agents pursuing targets in crowded environments. Players had to balance stealth, disguise, observation, and timing while trying to identify and eliminate opponents.

The single-player campaign continues Ezio Auditore's story and is set primarily in Rome. It also expands the idea of recruiting and training Assassins, allowing Ezio to call on members of his Brotherhood during missions.

Earlier Assassin's Creed games focused on single-player historical adventures, while later entries changed or omitted multiplayer in different ways. Brotherhood is therefore remembered both for its Rome campaign and for making online multiplayer a prominent part of a mainline release. Its multiplayer systems were separate from the cooperative mode later used in Unity.