Series three regular cast members include Colin Morgan, Bradley James, Katie McGrath, Angel Coulby, Anthony Head, and Richard Wilson, as well as Emilia Fox joining the regular cast.

John Hurt took his previous role as the voice of the Great Dragon but was no longer one of the regular cast (though he continued to provide the opening narration for the series).

Now in league with Morgause, Morgana's powers begin to grow and she becomes a deadly enemy within the walls of Camelot, but with Uther and the kingdom blind to her treachery, can Merlin thwart her plans before she can destroy Camelot?

Old friends return to the kingdom, and new enemies grow stronger outside the walls of Camelot.

Merlin must be more alert than ever if he is to protect Prince Arthur, for his greatest enemy is now within the castle walls....Morgana.