Well, 'The Unholy' is purely fictional. It was crafted by the imagination of the writers and not inspired by any actual happenings. The story and its elements were made up for entertainment purposes.
In most cases, 'Unholy' is not rooted in real events. It's probably the product of the creator's imagination and creative process. There's no evidence suggesting it's based on actual happenings.
One popular game - related novel could be 'Sword Art Online' which has some elements that can be related to the idea of a game in a novel form, though it's not hentai. Hentai involves inappropriate and often sexualized content which is not acceptable in mainstream media.
Most of them are designed with adult themes in mind. They may include explicit scenes, so they are restricted to an adult audience. However, there are some visual novels with similar art styles but without adult content that can be enjoyed by a wider range of people.