The Monster Lord's Castle is a location in Monster Girl Quest Paradox
Located in Hellgondo and originally home of Alice the Sixteenth, this place has been the seat of turmoil as the White Rabbit infiltrated the place and sealed the Monster Lord before the beginning of Luka's adventures. When the now sealed Monster Lord went on a lone chase to retrieve her power from the mysterious monster, her supposedly dead mother Alice the XVth returned and took over with the support of the Lilith Sisters before taking a slew of questionable decisions.
At the end of the second chapter, the party learns from a freshly recruited Morrigan that the Monster Lord has been killed by Marcellus. The ramifications of her death are yet to be explored, as the place hasn't been accessed yet.