Hime to In'yoku no Testament

In Winstol Kingdom's dark province of Woodfall—

A land where ominous clouds have sealed the sky, devoid of sunlight for a whole century, feared as a "cursed realm" forsaken by the gods, is governed by the so-called "Monster Lord," Gran Woodfall.

Secluded in a timeworn castle deep within an enshrouded forest, Gran is a bona fide monster, unaging due to a curse, who has lived for a century.

Summoned to this lair of monstrosity is Princess Sylphia of Winstol, offered as Gran's bride-to-be as per an unholy pact between him and the King of Winstol. She is the "cursed princess," confined to the royal castle for carrying a calamity-invoking curse since her youth.

But that very curse is the key to Gran's quest.

"Everything is in place.
Ring the chimes, mark the passage of time, and fill the vessel with the curse.
Now—let the 'ceremony' commence."

Gran has lived through a century, fueled only by the desire to reclaim his lost love taken by an unjust fate. Having gained the necessary power from Sylphia, he commences the 'ritual' alongside the condemned monsters he has summoned to his castle.

In a forest shrouded in mist and darkness, the curtain rises on a dark, sensual rite within this cursed citadel—.

[Translated from official website]