Ok so how about this? Let's say that the Holy Tomb ritual worked. Rhea got Byleth in position, and the moment Byleth sat down on the throne, that was the day Fodlan's fate was changed. Byleth's soul was taken out of her body and switched with Sothis, but it wasn't like a quick process. No, it was a PAINFUL experience. Byleth felt her soul, her very life force being extinguished, tortured, burned as though in hellfire, screaming in horrendous pain as Sothis tried to calm down Byleth, terrified and worried for her as she felt her soul being forcibly pulled into Byleth's body. During the process, Sothis regains her memories and is finally in control of Byleth's body while her soul has...vanished. Rhea is ecstatic while the class is worried, but Sothis wakes up and is absolutely furious at Rhea. The first thing she tries to do? Attempt to kill her own daughter. Rhea is too shocked to fight back, only defending herself as Sothis attacks with both the sword and her magic, screaming at Seiros and demanding what horrible thing she has done, calling her a monster. Sothis tries to finish off Rhea but feels her body being pulled back. She realizes this must be Byleth's doing and is relieved there is a chance for her to return. She lets Rhea live for one reason to force her to get Byleth back. Now, she pretty much hates Rhea for many reasons; violating her teachings, being too obsessed with her, "brainwashing " her followers, making her a Goddess when she didn't ask for it, twisting the truth of Nemesis and the Crests, playing God with other lives, Byleth, and much more. At best, she disowns Rhea, berats Seteth, is probably only nice to Flayn, and is willing to do anything to get Byleth back, even sacrificing herself to Edelgard and willing to be an experiment. She might even try and destroy the religion and churches. This was my idea for a possible story.