Successfully complete the indicated task to unlock the corresponding cheat 
under the "Cheats" option at the "Abstergo Challenges" menu.
Note: Enabling a cheat will prevent the game from being saved.

Armed To The Teeth: Successfully complete 90 Abstergo Challenges.
Arrr Matey!: Successfully complete 20 Abstergo Challenges.
Celestial Navigation: Successfully complete 10 Abstergo Challenges.
Dead Men Tell No Tales: Successfully complete 70 Abstergo Challenges.
Deceased Crew: Successfully complete all 100 Abstergo Challenges.
Loaded To The Gunwhale: Successfully complete 50 Abstergo Challenges.
No Quarter: Successfully complete 30 Abstergo Challenges.
Poseidon's Will: Successfully complete 60 Abstergo Challenges.
Scourge Of The Seven Seas: Successfully complete 40 Abstergo Challenges.
Shiver Me Timbers: Successfully complete 80 Abstergo Challenges.