Thicker than Water 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 assassinate a conspirator with ties to Archbishop Francesco Salviati, one of the Pazzi conspirators.
Ezio followed the gondolier to the destination and assassinated the target, completing the contract.
Ezio following the gondolier
Ezio assassinating the conspirator