The ancient device sought by Edward Kenway in Assassin's Creed IV: Black Flag is the Observatory.
The Observatory is a First Civilization site in the Caribbean that allows its user to observe a person through a special crystal skull linked to that individual’s blood. Templars seek it because the device could provide extraordinary surveillance power.
Edward becomes involved in the search through the pirate and Templar conflict surrounding the Sage Bartholomew Roberts. The hunt leads him across islands and into a struggle over whether the Observatory should be controlled or protected.
The Observatory is different from an Apple of Eden, another type of powerful First Civilization artifact appearing throughout the franchise. Black Flag also includes a modern Abstergo storyline, but the Observatory itself belongs to the historical Caribbean plot.