Cardinal Sin 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 Niccolò Machiavelli to assassinate a Borgia cardinal.
Ezio assassinated the cardinal, demonstrating that the church could not protect the Borgia followers.
Ezio locating the cardinal
Ezio assassinating the cardinal