A Fools Vow

Only A fool would bargain the life of their first born son for power. No one can be that greedy right? Obviously Uther didn't get that memo when in a moment of weakness he makes an unbreakable vow to give up the life of his first born son, to a demon so he can become king. But what happens when said demon no longer wants to kill the crown prince? (i do not own Merlin)