The fantasy drama, starring Colin Morgan and Bradley James, will return to BBC One early next month.
The new 30-second clip gives viewers their first glimpse at new regular character Agravaine (Nathaniel Parker) and also showcases the return of the Great Dragon (voiced by John Hurt).
Other stars confirmed to appear in future episodes of Merlin include Whitechapel's Phil Davis, former Spooks star Gemma Jones, Battlestar Galactica's James Callis and Rome's Lindsay Duncan.
The show's co-creators Johnny Capps and Julian Murphy recently revealed that a film version of the hit series is also in the works.
Watch the new Merlin trailer below: