The show will end after five series later this month with a special two-part story called 'The Diamond of the Day'.
The trailer shows Morgana (Katie McGrath) preparing for war on Camelot, with a vengeful Mordred (Alexander Vlahos) at her side and appearing to forge a sword with Aithusa's breath.
A series of images from the final episodes were released by the BBC in conjunction with the announcement that the show was coming to a close last month.
Speaking about the show's upcoming end, Colin Morgan (Merlin) said: "From the beginning, this was always going to be a five-year journey that we embarked on, and I think the show has run its natural course.
"The show has grown and grown each year and now we've arrived at its strongest point and we've achieved what we set out to do... I know this is the end, and I know this is goodbye, but thank you for being there on the journey with us because it has been a lot of fun!"
Merlin continues on Saturday, December 22 at 7.55pm, with the last ever episode airing on December 24 at 8.15pm on BBC One.