Chisa na Ou no Tanetsukegokko (Little King Breeding Game)
The Emperor of hentai/doujinshi fantasy land is quietly assassinated by his wife of the past 6 years. She does this not to become queen herself, but for the sole purpose of crowning her grossly-underage son emperor and legitimising her obscene sexual relationship with him under the pretence of stabilising the empire. Before this can happen, rulers from lands closest to the empire impose that a wife for the child be selected from a much larger group of adult women willing to marry a 5 year old: all of whom just so happen to be just as depraved and disturbed as his mother.