Cube (冥界島, キューブ, Kyūbu) is a floating island housing prisoners.
What used to be serving as home to Mages of Tartarus, Cube is a massive, buoyant cubed-island that harbors several rocky terrains spread about on its surface. The largest structure is, undoubtedly, one of its many landscapes accommodating Tartarus' headquarters. Due to its ability to float about freely, Cube's exact location and coordinates are unknown to many, including the Magic Council back when first created.
Now, the Magic Council has utilized the Cube to become the official Magic prison for caught Dark mages and others who were arrested by the Magic Council