The action-packed trailer features swordplay, epic battles, dragons and sorcery to the tune of Muse's 'Knights of Cydonia'.
Show co-creator Julian Murphy has previously dropped hints that this series will feature a much "stronger, focused and [more] powerful" Merlin than in previous runs.
Murphy said: "I think it's interesting because you saw a glimpse [of that] at the end of the last series, particularly when he killed Agravaine.
"We're really interested in taking him on that journey with all its dangers and the temptations it will bring, so that will be a big part of the fifth series."
Co-creator Johnny Capps previously told Digital Spy that he and Murphy had always planned out five series of the show, but that there is now "every possibility" it could continue into series six.
The fifth series will air on BBC One in the autumn.
Watch the new trailer for Merlin below: