Cube (冥界島,キューブ, Kyūbu) is a floating island housing Demons of Tartarus.
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.
Cube's appearance after Alegria is used
As a result of Alegria, Cube is vastly changed in both appearance and size. The outside of the floating island expands dramatically, with the guild building's earthy islands moving to create almost decorative patterns on the surface of the mass, whilst scaly protrusions also begin to appear. On the lower face of Cube, these protrusions take on the form of long, plated tentacles and dark spines, which hang downward and appear as legs. On the top of Cube, the new protuberances look more like the fins of a fish, and flare out on either side of the island almost like hair. In addition, crag-like formations rise from the top of Cube, creating large cliffs that jut from the top of the structure. However, most notably, a section of the lower face of Cube breaks away completely, opening up one of the side faces to resemble a humongous mouth with over-large teeth and fangs. As an entirety, the building resembles a huge Demonic creature and also takes on a new name: the Plutogrim (冥王獣 プルトグリム Purutogurimu).
The inside of the Tartarus headquarters is also altered, with the previously castle-like hallways and rooms being completely melted away. Along with the warping of the different locations in the hideout, anyone present will also be consumed, being dragged down into the melting rooms as the change is under-way. After some time, the liquefied building begins to harden over, re-solidifying and trapping anyone unlucky enough to have been caught up in the transformation.
Despite the grim effects of Alegria, it can be noted that surviving the transformation of Cube is possible, though there is apparently only a one in one million chance of this occurring.