Responses
Don't wanna give spoiler, but you're almost right and at The right lane
Oooh exciting!!!!
I had assumed that yurigel is using the prince as a vessel of sorts and when he gets old enough, yurigel is gonna abandon his dying body and take over the body of the prince. But ash may be acting as a strange variable since she doesn’t have the mindset like the others in that world.
So recent chapter "revealed" (or maybe I just forgot) that Yurigel is part of house Korterin. And then Ciel said that house Korterin has produced many great mages in the past. I'm assuming the magic circle they are looking for is the circle that is used to get the Queen pregnant. And the locket they found has a boy/baby/kid?? with the name Michael Korterin on it. The prince's name is Michael.
So MY theory is that Yurigel probably got his grubby ass hands on the King and then made the Queen give birth to the prince, using the magic circle that could create great mage, to make the second coming of Michael Korterin. And that's why Ash is in his way.
But hey, that's just a theory, a manhwa theory ╮(￣▽￣)╭