In Assassin's Creed Brotherhood, Ezio holds the title Il Mentore after becoming the leader of Rome's Assassins.
The Italian title means “the Mentor.” Ezio takes the role after rebuilding the Roman Brotherhood and organizing resistance against the Borgia regime. As Il Mentore, he recruits and trains new Assassins, coordinates missions, and directs the order's operations from Monteriggioni and Rome.
Brotherhood continues Ezio's story after Assassin's Creed II. Its main setting is Rome in the early sixteenth century, where Cesare Borgia and his allies control much of the city's political and military power.
The title is not the same as “Grand Master,” which is commonly associated with the Templar hierarchy. “Il Mentore” specifically identifies the senior leader of the Assassin Brotherhood in the Italian Renaissance storyline.