Stheno being able to claim Celestia as a thrall is definitely an XK-class event, he would be able to alter celestia's solar manipulation ability so that it causes the sun to instead give off gamma radiation instead of the UV light that negatively interferes with his abilities. imagine the "daybreaker" SCP but the light turning everyone into hulk monsters instead of hostile goop in that case.

It's very unlikely to happen since she's got a lot of things going for her to keep stheno from infecting her (I imagine radiation resistance being one of them) but with sufficient power he could overwhelm her and you'd have to do time travel shenanigans to un-fuck things (since stheno is a chaos being though the whole "he's already there" would go into effect if you attempt a reversal of causality at any point that it would stop him, but at least then it would be a 1v1 confrontation). I would love to kind of see this drawn out, though I kind of feel there are more pressing matters at hand that I need to attend too to get too bogged down in the fanfic stuff lol

ANYWAYS thanks to faogwolf! I really appreciated his creative input here, I pulled up a 1950's monster movie poster/book cover or 2 with Frankenstein or Dracula doing the whole bridal carry thing, but he went with a kind of Dracula from the "Van Hellsing" movie during the vampire masquerade dance, and while I was a little skeptical at first it did work out pretty well and seemed rather in character.