Imprison the young king
From Create Your Own Story
You order the king's own guards to imprison him. Since the guards fear you far more, they actually do this. However when they find their king, he has performed a summoning spell. He has spent the last five years, secretly learning magic.
The summoning spell compels someone to head to the location the spell was cast, your castle.
The king is imprisoned but reveals that the thing he has summoned is your brother who will surely attack when he arrives.
