Erotical Wizard to 12-nin no Hanayome

Junpei Ichinose, who attends Tokyo Magical Academy, could no longer prove his magical aptitude. Out of nowhere, a beautiful witch named Meryl appeared in front of him and said: "You, the reincarnation of the 'Maou,' are needed for..."

Junpei is now the only person who knows that he has the power of the demon lord. Furthermore the information leaked to the beautiful Sonia, who is a descendant of the hero. Junpei doesn't know how to use the demon lord's power and manages to convince Sonia to help him find the real way to use it. But as it turns out, the demon lord's magic power is called "Ero Magic." Thus begins the battle harem story with beautiful girls and the "Ero Magic" spell!!

(Source: Hobby Japan, translated)