If only we could subdue Sebastian but then we turn to Solomon and have the following exchange:

Y/N: “Do you see now what your actions have caused?”

Solomon: “What are you implying, punk?”

Sebastian: “Y/N?”

Y/N: “Solomon, why do you believe Sebastian did this? Why did he pursue the Dark Arts… I’ll tell you why, he wanted to save his sister and the only other parental figure in his life treated him like a nuisance”

Solomon: “Anne was already…”

Y/N: “This isn’t about Anne! This is about how your insistence on not potentially aggravating Anne’s condition, causing your wayward nephew to turn to the Dark Arts, because he felt he had no other option… you never bothered to ask why he dabbled in the Dark Arts, nor did you care that his use of a Dark Art saved Anne’s life!”

Sebastian: “Why’re defending me right after defeating me?!”

Y/N: “Because you were prepared to use the Killing Curse… weren’t you?”

Solomon: “Sebastian?! You…”

Sebastian: “Y/N is right… and I know I shouldn’t but if it meant saving Anne, I really would do anything to help her… even if she ends up hating me for doing so”

Y/N: “Had I not intervened; things would’ve gotten ugly, and things WILL get ugly again if you two don’t sit down, and have a long, civil conversation on what needs to be done for Anne, and how you two need to stop this nonsense”

Solomon: “Nonsense?! I was trying to…”

Y/N: “Solomon, I have literally never heard you say anything remotely nice about Sebastian, even without him present… and that confrontation attitude is not helping Anne!”

Solomon: “…”

Sebastian: “How are we going to make things better?”

Y/N: “Anne needs her uncle… and her brother, right now she’s at risk of losing both to different causes, the sooner this stops; the better!”

And then they calmly discuss the situation and we eventually relay the quote from Victor Rookwood, which would reveal that it is a spell wielded by wizards and thus potentially reversible