Isekai NTR ~Shinyuu no Onna wo Saikyou Skill de Otosu Houhou~ - Vol. 3 Ch. 19 - The demon lord castle

[MangaDex]

 

[Ore]

So the Demon Lord castle is a Series X.

Pffft...

 

[Otherworlder]

It looks like the developers never got around to finishing this level. It's a big black box containing a single empty room, with [placeholder boss here] with infinite hit points, intended to be temporary until they got around to actually designing his capabilities.