Merlin Season 3 Episode 5

Fleeing enemies Merlin and Arthur take refuge in the Crystal Cave where magician Taliesin cures the wounded Arthur and shows Merlin a vision of the future in which Morgana stabs a sleeping Pendragon. Come her birthday Morgana is given a dagger by Arthur and steals into the king's room,followed by Merlin,who uses magic to thwart her. She is temporarily put into a coma during which time Pendragon confides in Gaius that she is his natural daughter,a fact of which she is also aware. Gaius believes that Arthur should be told as now Morgana is a legitimate heir to throne and he stands in her way.