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.
I'm sorry I don't have exact details about 'cross x regalia light novel' as I haven't read it myself. But generally, light novels often involve elements like unique characters, a fictional world, and some kind of overarching plot, which could be about adventure, fantasy, or other themes.
I'm not sure specifically as I haven't read it. It could be about a night filled with dark or unholy events, perhaps some sort of mystery or horror story that takes place during the night.