S4, E3: The mighty citadel is buzzing in anticipation of the Prince's birthday celebrations. But Arthur's old adversary, Odin, promises to ensure this is one party Camelot will never forget. For amongst the assembling guests lurks a deadly assassin (Phil Davis) set to strike. As the dust settles, Camelot is left devastated. But just how far is Merlin prepared to go to put things right? Would he really risk everything and reveal his secret?