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).

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.

As the King's loving ward plays her game of lies and manipulation, can Merlin stop Morgana before Camelot is lost forever, or is the kingdom set to crumble under the force of secrets and lies?