Found it! Merthur magic reveal in other kingdom with crazy king
Pls send me the link. Merlin, prince Arthur and knights are sent to another kingdom, cursed. Many people have left to protect their children from some curse/spirits. They hope prince Arthur will go there to take over and save their people.
The local king surrounds himself with children as when the spirits attack, they make people age. There are now lots of spirits around as they cannot cross the veil. Also, in the story, Leon is the first to suggest Merlin may be magic. Prince Arthur also interacts with a little girl with magic who sneaks in his chamber sometimes, and whose mother is one of the spirits, or their leader. Merlin or Arthur are accused of magic and condemned. Happy ending. I do not manage to find it anymore.
Thanks