A Tsukuyomi/Setsuna story that takes place in the same continuity as this one
. ...That kind of makes it a prequel, so reading it would be a good idea. It doesn't matter much for this first part, though, so feel free to ignore if you aren't interested.
This is just the prologue, and there will be multiple chapters assuming I ever get around to finishing it. It takes place shortly after the current arc, and I'm pretty sure that some future chapter will prove that it's AU, but for now I will happily embrace my delusion that something like this could possibly happen.
Thanks for reading!