Skeleton Key

The Skeleton Key, named Bone in the original release, is a key item that appears in Final Fantasy Adventure and its remake, Adventures of Mana.

Final Fantasy Adventure/Adventures of Mana
After Sumo slays the Marilith in the Cara Mountains, Lorim's curse is lifted and its populace freed from their icy grave. There the hero reunites with the sage Cibba, who entrusts him the Skeleton Key said to unlock the door to the Undersea Volcano housing the legendary blade.