Undynecest NSFW/Smut

This is a not very common cest ship in the Undertale fandom. I mean a lot of people like or love Fontcest, Papcest, and Sanscest. I even saw an Alphcest one somewhere! though I don't exactly remember where. and I decided to make an Undynecest one shot book. and it's gonna be veeerrrry NSFW, Lemon, Smutty or whatever e...