Not the Royal family. It was one of the families of daimon empire who had powers like summoning, exorcism, barrier, future sight etc.
So the obvious culprit is the one summoning spirits and asking them to latch onto roderick. To harm him from the inside. The Royal prince supposedly met him while studying abroad. And well, he unilaterally decided that the blake royal family won't be able to stop him cause he's not alone. There's another guy. Mellon and Russell, these two. So he chose to protect what little he could even if it meant sacrificing his brother. He thought that roderick won't be able to stop them and die, then the people will suffer. So he sided with the enemy to protect them.
He's not completely wrong, but even he couldn't have imagined Hailey would also aweken her power and choose to die so that lee jae could come here. There's also the boy king. And this changes everything.
More importantly, he did not trust his brother at all. He chose to believe in the enemy rather than his family. Well the plan failed and he got executed as a traitor but that's a thing for later. But i was a little surprised that the author did not give him any closure at all. His ending(death) was swept up in the rug lol. Like one sentence "all the traitors got executed" and the royal prince was included in that but the author did not point him at all. Like a no name extra he was gone.
And you know what hit me so hard, the fact that roderick said he would've understood him if he did it for the throne but that wasn't it. Roderick probably felt hurt by his own brother's decision but he didn't let it get to him. He is a king after all and it's his duty to protect his people and land. He's such a good guy i swear i get a little jealous everytime i read this.

iirc, Roderick was cursed by someone from the royal family. Was it this Royal prince he mentioned? Its his brother right? Whats his reason for cursing our dear Roddy ( ̄へ ̄)