Leader of the Pack was a virtual representation of one of Ezio Auditore da Firenze's genetic memories, relived by Desmond Miles in 2012 through the Animus.
Ezio Auditore received a contract from Lorenzo de' Medici to find and assassinate the leader of the condottieri who have attacked Palazzo Medici.
Ezio assassinated the target, completing the contract.
Ezio following the guards
Ezio assassinating the guard leader