Merlin - Season 4 Episode 6
A Servant of Two Masters

Merlin becomes a deadly weapon in Morganas fight for supremacy when he falls into her hands. By recurring to ancient magic, she manages to turn friend against friend and creates the perfect assassin out of the young sorcerer which puts an oblivious King Arthur in great danger. Will anyone notice Merlins unusual behaviour before he is able to do any serious harm to the King?