Merlin - Season 5 Episode 6

After visiting her father's grave, Guinevere is captured by Morgana and imprisoned in the infamous Dark Tower haunted by ghostly visions and screams. Merlin and Arthur lead the knights in a rescue mission but get lost in the thick and aptly named Impenetrable Forest. Mab, the fairy queen of this forest, appears to Merlin and shows him how to guide the others through it but tauntingly reveals that one will not return. Arriving at the tower, they save Guinevere but at the cost of a man. None, however, realise that Guinevere is a changed woman thanks to Morgana.