Emrys (Series 4)
played by Colin Morgan
Old Merlin, otherwise known as Emrys, is Merlin's alter ego. The young warlock uses an aging spell to transform himself into an old man and must take a potion to transform back to his young self. Gaius is the only one to know Old Merlin's true identity. Morgana is terrified of the aging warlock as she knows him to be her doom.