Merlin - Season 5 Episode 3

Arthur rescues an old lady accused of sorcery, who gives him a horn with which to summon the spirit world. Despite Gaius's warning, he uses it to briefly meet his father but soon realises his error as Uther terms him weak for his non-political marriage, his practice of knighting ordinary young men on merit, and his lack of cruelty to make his subjects fear him. When Camelot falls prey to poltergeist attacks, with Guinevere the principal victim, it becomes clear that Arthur has unwittingly released his disapproving, vengeful father's spirit on the court. With a potion made by Gaius to make the spirit visible, and with Merlin's help, he sets out to return Uther to the grave, delivering a few home truths before it goes.