A Templar key was one of a set of five keys which together granted access to a vault in the manor on Great Inagua during the 18th century.
Each key was entrusted to a Templar stationed in the Caribbean: Julien du Casse, Hilary Flint, Lucia Márquez, Kenneth Abraham, and Jing Lang. After the pirate Edward Kenway delivered a map showing the locations of Caribbean Assassin bureaus to the Templars, the keyholders, excluding du Casse, were each assigned a bureau leader to execute.
When Edward assassinated du Casse and procured the first key, he discovered the vault it unlocked contained a desirable set of armor. Upon finding a map showing the locations and missions of the keyholders, he agreed at Mary Read's insistence to warn the targeted Assassins, provided it led him to the other four keys.