Deral is a visual riff on Dural, the final boss of SEGA's Virtua Fighter games.
When the game was shown at Tokyo Game Show 1997, only a simple non-textured arena featuring two tiers and slopes between them was available [src]. Credit goes to Alex's Bomberman SS Shrine for the information and screenshot.
SEGA Saturn Magazine Vol.34's coverage is all on this unseen stage set on a four-tiered 14x14 pyramid, that resembles nothing in the final game. Even its background isn't used anywhere!
The same issue features a slightly different character select screen; Kurobon's icon is facing the wrong way, and many of the character icons are cruder-looking, including Baguro, Combatant #12, Golem Bomber, Pegi Jr., Radibom, Denta and Pump. It's hard to see, but the backdrop is cropped from promo art of Saturn Bomberman.
By editing the stage select value to 10, 11 or 12, you can access three stages that are otherwise totally inaccessible.
They load the incorrect textures in the stage select preview, but are otherwise fully functional and even have unique graphics for their names (Bonus Zone / おまけその). They deal primarily in slopes and elevations, often to the detriment of actual combat. They're even acknowledged by the official guidebook!