2x27 - The Legend of Prince Valiant

A recurring nightmare involving a golden serpent has the prince worried about his future and the safety of Camelot. Merlin tries to reassure him that some dreams are just dreams and sends him on a mission to see The Mighty Oom. While there, Valiant's dream grows more vivid and deadly - he can feel that something bad is coming. What eventually does come threatens Camelot as well as a lovely young woman Valiant meets.