Merlin - Season 4 - Eps 1: The Darkest Hour (1)
Evil sorceress Morgana summons mighty gatekeeper the Cailleach, who opens a door to another world from which hellish creatures pour forth. As Arthur and his loyal knights take up arms, Merlin discovers the only thing that will stop them is an unimaginable sacrifice.
About Merlin

Title: Merlin
First Air Date: 2008-09-20
Last Air Date: 2012-12-24
Status: Ended
Rating: 7.8/10 (from 1001 votes)
Language: EN
Seasons: 5
Total Episodes: 65
Network: BBC One
Genres: Action & Adventure, Drama, Sci-Fi & Fantasy
Production Companies: Shine TV
Synopsis
The unlikely friendship between Merlin, a young man gifted with extraordinary magical powers, and Prince Arthur, heir to the crown of Camelot.
Cast

Colin Morgan
Merlin

Angel Coulby
Gwen

Bradley James
Arturo Pendragon

Katie McGrath
Morgana

Richard Wilson
Gaius

John Hurt
Kilgharrah (voice)

Tom Hopper
Percival

Rupert Young
Leon