Each of the Item-slot items dropped by the Avatars which are used in the Mama Mia quest were a Key Item from one of the first seven Final Fantasy games. Ancients' Key was a key item in Final Fantasy VII required to activate a device in the City of the Ancients, which would reveal what happened to the White Materia and whether Holy would come when Sephiroth & Jenova were dealt with. The key was located in an underwater cave located beneath the northern continent, requiring a submarine to navigate to.