“glowfic” apparently. written in a roleplay forum format.
This is not a story for kids, even less so than HPMOR. There is romance, there is sex, there are deliberately bad kink practices whose explicit purpose is to get people to actually hurt somebody else so that they’ll end up damned to Hell, and also there’s math.
start here. or don’t, of course.
It’s actually kind of an achievement that in the ultra-saturated Isakai genre they’ve managed to set a new standard in repulsive premises.