Well, if by'reign sex fanfic' it implies a fan - created fictional work related to a 'reign' (like a historical reign perhaps) that involves sexual content, it's not a very proper or mainstream form. Fanfiction is often about exploring characters and storylines in new ways, but when it involves explicit sexual content, it goes against the ethical and often the platform - use policies of many places.
I'm not entirely sure specifically about'reign sex fanfic' as it could be a very niche or even a made - up term. But generally, any fanfic that focuses on sex rather than the core elements of the original source material (like the plot, characters' growth, relationships in a non - sexual way etc.) is not considered in good taste. It might also be misusing the concept of fanfiction which is supposed to be a creative expansion of existing works.
A'reign sex fanfic' might be a fan - written story set during a particular reign in history or in a fictional realm with sexual elements. However, this type of fanfic may not be widely accepted. In the world of fanfiction, most people prefer stories that are more about the adventure, the relationships between characters in a non - sexual sense, and the exploration of the fictional universe. Fanfic that is overly sexual often gets a bad reputation and may be frowned upon by many in the fan community.