In Assassin's Creed Brotherhood, what feature was introduced to the main series for the first time?

Assassin's Creed Brotherhood introduced online multiplayer to the main Assassin's Creed series for the first time. Its multiplayer mode launched alongside the single-player campaign in 2010 and placed players in competitive stealth matches.

Instead of controlling a single historical hero, multiplayer participants selected trained Abstergo subjects who used different Assassin and Templar personas. Modes such as Wanted and Manhunt rewarded careful observation, disguise, pursuit, and precise kills rather than ordinary fast-paced combat.

Brotherhood was not the first Assassin's Creed game to feature every later series element. Naval warfare became a major system in Black Flag, playable women appeared in several forms across the franchise, and role-playing mechanics were greatly expanded in Origins. Brotherhood's specific milestone was online multiplayer in the main series.