18+ Doreigaeshi no Hangyaku Maou: Saijaku Shuzoku no Akuma ga Kindan Harem wo Kizuku made

100 years ago, the war between humans and demons ended with a victory for humanity.

Defeated, the demons' numbers dwindled, and survivors were overwhelmed by the humans under the protection of the Goddess. Since then, demons have been destined to become human slaves at birth, and it's become commonplace for them to spend their entire lives serving humans.

Liebe was reincarnated into such a world as a demon. He had died in an accident, driven to despair from bullying. However, he was reincarnated into a world where demons were treated poorly. Liebe was also made a slave (pet) to the daughter of a frontier noble, and spent his days serving her. Even though he wanted to fight back, his slave contract prevented him from doing anything.

However, Liebe possessed special eyes that allowed him to see his opponents' weaknesses. Combining this power with magic, he developed a spell that temporarily repelled the slave contract. Since he'd gone to all the trouble of becoming a demon, he decided to become the hope of the demons... and become the strongest demon king!

(Source: Futabasha, translated)