18+ Grimoire no Shisho: Majin no Miko to Ubawareshi Danpen

Magic exists in the world of Grand Esteca. There’s a magic library in the town of Altrium where all books of sorcery are kept. There resides a closely guarded book 30 floors underneath the library: the powerful book of sorcery, Grimoire, within which was sealed the demon who unleashed his fury in a war long ago. However, now he is merely a perverted demon with no dignity whatsoever. The librarian Mihai loves reading books and came upon and became friends with Grimoire. He respected the demon despite his current situation due to his immense magical knowledge. However, one day, someone stole these pages of knowledge from Grimoire and they ended up dispersed in other continents. It might cause a new world war to break out. Concerned about the situation, the sage Maynard ordered Mihai to recover the pages.

(Source: Hau~ Omochikaeri!)