FE:3H - Dimiashe
In the monastery there are a lot of spirits, like Howards level, and there is this one that suddenly started following Ashe. He, because he's Ashe, doesn't mind, but Sothis, being a god and all, is like "That's suspicious, do something about it"
So Byleth starts following Ashe to check on him, and Dimitri notices the teach doing his thing and is like "Hey professor, what gives?" Byleth explains that he is suspicious about the spirit. Dimitri of course starts helping checking on his crush.
The spirit doesn't talk, Ashe tries to befriend it but it just stares at him.
So time passes, just like a month, and then one day in class the frickin spirit suddenly touches Ashe's heart. Like, literally, his hand goes through Ashe's chest and touches his heart. Of course everybody freaks out but Ashe is like "Guys I'm ok I just need water" so he stands but his legs get all jelly like and collapses.
Mercy, Ann and Ingrid go help Ashe; Sylvain, Dedeu and Felix chase after the spirit who just runs away/disappears; Ingrid goes for Manuela, Flayn goes to tell Sethet and Dimitri is in the verge on going feral for obvious reasons and doesn't know if he should check the condition of his love or chase after the spirit for revenge, so Byleth sends him with Flayn to notify Rhea and then goes with the rest of the boys after the spirit. They don't get the spirit and go to the infirmary where the rest of the class already is. Also Jeralt is there, and Ashe is already conscious. Manuela says that the spirit put a curse in Ashe, but she can't identify it so they don't know what it does, making it the more dangerous.
Dimitri is having a breakdown.
Then Rhea calls for Byleth and tells him that their mission is to find the spirit and break the curse.
The curse is about his heart rotting and slowly losing his feelings, and all his being slowly shutting down.
Truth is, the ghost is in love with Dimitri and the reason he spent so much time with Ashe is because he noticed that Dimitri watched Ashe with a gentle expression and wanted to see what kind of person Ashe is, and since he is so nice he felt even more jealous and cursed him.