Episode 11 of 13. First transmission on Saturday December 5, 2009 (UK)
The official details about the episode are as follows…
The ruthless warlock Alvarr is determined to destroy Camelot and allow magic to reign again, as the magical family drama continues. To fight Uther, however, Alvarr must reclaim the Crystal of Neahtid – a precious magical relic of the Old Religion. Few can harness its power, but Alvarr believes Mordred can use it to further their cause.
The crystal is safely locked up in Uther’s vaults and many have lost their lives trying to reclaim it. Alvarr, though, has a secret weapon – Morgana’s powerful bond with the druid boy. Morgana has always protected Mordred but will she go as far as turning into a thief and traitor to help him and the charismatic Alvarr?
Merlin is shocked by her choice and by the true power of the mysterious crystal.
Alvarr is played by Joseph Mawle, Uther by Anthony Head, Mordred by Asa Butterfield, Morgana by Katie McGrath and Merlin by Colin Morgan.