One of the NPCs at the camp in Eureka Pyros talks about it, but it's just vaguely believed to be the same as its FF III counterpart: a labyrinth filled with ancient and powerful weapons.

On an unrelated note pertaining to the actual difficulty of the dungeon, the fact that enemies no longer crit in ShB onwards is really significant for solo runs because you can easily die from bad crit RNG in both PotD and HoH on the later floors.