‘The Darkest Hour’ airs at 7.50pm on Saturday 1st October and sees Merlin face his toughest challenge yet when Morgana’s blinkered determination threatens not only Arthur’s future, but also the very balance of the world.

Series 4 Volume 1 will be released on DVD on Monday 28th November.

> Order the Series 4 Volume 1 DVD on Amazon.

Series 4 Volume 2 and the Complete Series 4 boxset will be released on Monday 23rd January 2012.

The boxset’s exclusive bonus features will include a ‘making of Series 4’ documentary, cast and crew audio commentaries, out-takes, deleted scenes and ‘behind the scenes’ photo gallery and wallpapers for your computer. Each DVD will also include a top trump card exclusive to the DVD.

Watch the Series 4 trailer, with an exclusive introduction by Nathaniel Parker for CultBox…