Aku no Taizai: Venomania-kou no Kyouki

Duke Sateriasis Venomania is the only survivor of the massacre that befell his household—a massacre that he himself perpetrated.

Ready to end his life, Sateriasis is stopped by a mysterious figure known only as IR. Should he wish to change his fate and gain the affection of those who had previously tormented him, he need only stab himself with the katana that she provides. By doing this, he would make a deal with a demon, granting him the power to charm any woman he desires.

He uses these abilities for his own pleasure, building a harem of women to console him through his descent into madness. But when he meets Gumina Glassred, his childhood friend, he is reminded of the painful years spent under her scorn. With thoughts of revenge, he makes Gumina one of his captive followers. As his newfound demonic powers strip him of his humanity, nothing satisfies his carnivorous lust, and Sateriasis seems to have almost become a demon himself.

But, of course, where there are demons, there are those who wish to slay them.