“War has begun,” warns Arthur in the new trailer for the Merlin finale – and judging by the scenes that follow, he's right.

The evil Morgana is on a mission to steal Emrys's magic, thereby leaving the young wizard and his king vulnerable: "Arthur is nothing without Emrys and Emrys is nothing without magic." 

Watch the teaser below – and read spoiler-free synopses here – ahead of part one of Merlin: The Diamond of the Day on Saturday 22 December at 7:55pm on BBC1...