‘The Darkest Hour’ airs at 7.50pm on Saturday 1st October on BBC One and sees Merlin face his toughest challenge yet when Morgana’s blinkered determination threatens the very balance of the world.

> Order the Series 4 Volume 1 DVD on Amazon.

In this clip, Merlin freezes when something catches his eye in the corner of the King’s chambers, fearing it to be one of the Dorocha…