It was crucial for Lady Odette to know what would and wouldn't work against their Alpha King if she ever decided to run away from him.
Ymir didn't know when that time would come. He just wanted Lady Odette to be well prepared for the worst-case scenario.
"For example, if I use an invisibility spell on myself. You and Ruru won't be able to see me, because I have greater strength in terms of magic, at least for now. By the same logic, if I use it against His Majesty, he will still be able to see me perfectly," Ymir explained.
"Does that mean nobody can use the invisibility magic against His Majesty?" Odette asked as she caught up quickly on the hint.
