Since the kny hyperfixation has returned in full force, I had a thought– angsty one
Kakushi!Yui later finding out about the reprecussions of getting the slayer mark and getting very upset/pissed about it. Because in his mind, he's just thinking about Muichiro being taken away from him again and not being able to do anything about it.
Additional thought is like... (SPOILERS)
Muichiro not even living that long bc of the final battle and Kakushi!Yui being completely distraught because he pretty much had just gotten Muichiro back. Yuichiro would be in absolute shambles and I don't think there would be anything that could comfort him. It'd probably be a long time before he stopped mourning so deeply everyday. Just thinking about it makes me so sad for him.
I mean, I'm sure before that, he'd have warmed up to Tanjiro a little so I'm sure Tanjiro being the sunshine boy he is wouldn't let Yuichiro wallow by himself and get consumed completely by grief.
