I Received a Prophecy That My Fiancé Would Change His Mind

I received an anonymous letter.
Enclosed is a prophecy that my friend, Lady Margaret has been possessed by the spirit of a ‘transmigrator’, and that she took over all the prominent bachelors of the empire, making a reverse harem.
According to the prophecy, my fiancé, Carian, will betray me and confess his love to her…
“You, if you cheat on me, I’ll kill you.”
“…Are you obsessed with me? I’m so excited!”
However, I cannot rely on will alone. It may be a prank letter but I took a few precautions just in case. I have to prevent the evil spirit who possessed my friend and protect my fiancé.
“What’s this? A physical rights memorandum?”
“Excuse me, Carian. If you look in this bracket, it’s read as a memorandum of free will.”
I explained further as I brushed off a scrap of thread attached to my skirt.
“It’s about stopping your actions in case of an emergency, and setting up some restrictions. I have no knowledge when it comes to seduction, but in case you are trying something weird, I have to forestall you.”
However, there is room for discussion for the person involved. I hesitated to add.
“What about the agreement to provide and use personal information to a third party?”
“To observe her for the time being and to be able to respond urgently in case of any abnormal events. The prophecy said she changed after she fell while walking in the garden.”
“Then this?”
Carian lifted up a piece of paper with a strange expression, as if he was barely holding back his laughter.
“Can’t you read? It’s a consent form for the exorcism ritual. We need to get rid of the evil spirits. Don’t laugh. This is the key!”
Will I be able to fight off Margaret and prevent my fiancé’s change of heart?