The palm strike that hit Dyon's back before the battle began was precisely from his father-in-law and Ri's own father. Emytheus may have been prepared to make contingency plans against Little Lyla, but what would he do against another True Empath that had been gone for over a hundred years?
Dyon learned of the potency curses could have the moment he forgot about the existence of his unicorn corpse puppet. He went through so many battles where it could have been greatly helpful, yet he completely forgot about it.
It was then he made a few contingency plans. Who better to see through tricks of the mind if not a True Empath? However, that was contingent of the True Empath, him or herself, not being compromised in the first place. This was where his father-in-law who was currently sitting within his crumbling inner world came in.
"You were planning for me… For over a hundred years…?"