Beastly

 

Failing to take Uther for herself after Igraine’s death, Nimueh casts a spell on him. By day, he is a cold King, but by night, he becomes a beast. Only true love can break the spell, and Merlin might be it. 

 

Unfortunately, it doesn’t seem too likely, considering Merlin is a sorceress and Uther is a magic-hating King.