Shin Megami Tensei IV: Demonic Gene

In the Gregorian year of 1492, the people of the Eastern Kingdom of Mikado live peacefully in a social caste system between the wealthy Luxurors and underprivileged Casualries. In the empire, there are samurai, composed of both classes, who protect the Mikado citizens from the threat of demons. Among the new samurai recruits is a hot-headed Casualry boy named Walter and his childhood friend Gina. With a frequent habit of getting himself into sticky situations, Walter is constantly bailed out by Gina.

One night, Gina arrives to stop Walter from confronting a strong demon, only for the two to get ambushed and dragged into a demon domain. Though they survive the attack and escape with minimal injuries, Walter soon feels unwell. However, no later is he sent on a new mission to investigate the appearance of "literature" that turns people into demons, and Walter's symptoms worsen as he begins to question the morality of his actions as a samurai.

Unable to resist the allure of the forbidden "literature," Walter breaks into the Monastery's vault to read the confiscated book. As he turns the pages, the foundations of the kingdom he has called home for so long starts to fall apart before his very eyes.