Erotical Wizard With Twelve Brides summary:

Junpei Ichinose, a student at a magic school in Tokyo, is a dropout who has never discovered his magic aptitude. Suddenly, the beautiful witch Meryl appears before him and tells him, "I need your power, the reincarnation of the Demon King. Junpei finds out that he has the power of the Demon King. He learns that he has the power of the Demon King, and his secret is revealed to Sonia, a beautiful girl who is a descendant of a brave warrior. Junpei, who has no intention of becoming a Demon King, persuades Sonia to teach him how to use his power properly. However, the Demon King's power is the strongest and worst "erotic magic"!The harem school battle to master H-magic with beautiful girls is about to begin!