Gakuen Sei Senshi Sailor Knight ~Seigi no Heroine Kanzen Seifuku Manual~

Present day Japan: Unknown lifeforms called "Mouma" appear. Conventional weaponry is useless against them; the only countermeasure available is the "Sailor Knight", female warriors of justice protected by holy gemstones. An ordinary student, Utsuroi Hayata, worships the Sailor Knights from afar whilst enjoying sexual fantasies centred on them. One day, as Hayata is playing an adult game with a heroines of justice theme, the Mouma princess, "Ebony Carrot" emerges from his monitor in the same manner as the stereotypical RPG adult game trope. Hayata initially panics, but Ebony Carrot proclaims Hayata is the right candidate to become King of the Mouma, and she vows to assist him in breaking the noble and honourable Sailor Knights into his sex slaves... [loosely translated from official product synopsis]