Now that Morgana knows Emrys's true identity, she sends a beast to Merlin's chamber that sucks the magic out of him. Merlin is despondent not only to have lost something so dear, but also because, when Morgana and Mordred attack a garrison near Camelot, Merlin knows the foretold time is near, and he will be unable to protect Arthur after all these years.
His outlook worsens when he hears that Arthur means to ride to Camlan - the place named in the prophesy as Arthur's grave - and fight Morgana's army there, before Morgana has a chance to kill citizens of Camelot. Merlin must travel to the birthplace of magic, the Crystal Cave in the Valley of the Fallen Kings, if he is to have any hope of restoring his powers. He tells Arthur that he won't be able to go to Camlan with him, clearly disappointing Arthur in a way that might never heal. Gwen, however, does accompany Arthur, wanting to spend whatever time they have left, together.
Gwaine's companion, Eira, who is actually a spy for Morgana, reports to her lady that Arthur is bound for Camlan and Merlin is bound for the Valley of the Fallen Kings. This confuses Morgana, but Mordred remembers that the Crystal Cave, said to be the cradle of magic, is in that valley. Morgana reaches the cave before Merlin and traps him inside while she flies to Camlan.
Merlin, who was badly hurt in his encounter with Morgana, tries to escape by clawing through loose rocks and crawling through tunnels until he falls, exhausted, on the stone floor. In a dream, he sees his father, who comforts him and tells him that he doesn't simply have magic - he is magic, itself, and what he is can never be taken from him. He simply must believe that he is, incontrovertibly, Emrys.
Later, he awakens, rejuvenated by the power of the cave and his own belief, and tests his magic by conjuring a simple, ephemeral butterfly. He then summons images of Morgana's camp and her army's maneuvers. Though it's the middle of the night, Morgana's army is on the move, treading a secret path that enables her to outflank the Camelot soldiers. Merlin sends this message to Arthur's sleeping mind, strongly enough that Arthur believes the dreamlike words, and sends a scout to find the path.
Merlin's father appears once more to cement Merlin's belief in himself and his destiny, and Merlin makes his way out of the cave. He cloaks himself in his familiar aging spell and flees as fast as he can to the battle of Camlan that has just begun on two mighty flanks of one deathly theater.