Criterias:

- Nightmare Moon must possessed only Celestia, Luna, and Twilight

- Nightmare Moon must not thought to herself as "I'm gonna do evil just because I'm an evil villian" or something two dimensional like that.

- The story must not totally revolved around Nightmare Moon having mental sex with the host.

Optional but encouraged:

- Nightmare Moon is manipulative

-Nightmare Moon is weaken and must relied on the help of the host to do stuffs

- story was set after the return of Nightmare Moon(not ancient Equestria)

Examples
https://www.fimfiction.net/story/10149/entwined
https://www.fimfiction.net/story/448810/twilights-nightmare

If you don't mind my self promoting, it's a major plot point in my: https://www.fimfiction.net/story/303160/synthetic-bottled-sunlight