Ryouki ~Midara ni Hibiku Fukushuu no Rondo~

The protagonist, Van Renborg, and his sister were nobles but trapped in the clutches of the king's desires, enduring days filled with shame and humiliation. Van's sister, driven to despair, commits suicide. Fueled by hatred, Van plots a coup but fails, ending up imprisoned. Consumed by resentment, he is unexpectedly visited by princess Eldia, offering Van a chance for revenge by asking him to overthrow her sister, Filia, who holds a higher succession right. Eldia promises to restore Van's status in exchange for his assistance, unaware that she is resurrecting a vengeful spirit that will ensnare them both in the chains of depravity. [Translated from DLsite]