It's not a fleeting moment either - this episode is a fine showcase for both Coulby and Edun, with the latter arguably getting more to do than in any episode since his series three debut.
'The Dark Tower' begins with Morgana (Katie McGrath) ambushing Gwen, her brother and his fellow knights with some magic snakes - not her most daring plot, or the show's best effects work, it has to be said. Thankfully it's just a blip - elsewhere this week, the CGI is up to Merlin's usual high standards, especially the epic exterior of the Dark Tower.
Upon learning that his lady love has been snatched, Arthur (Bradley James) leaps into action, with Merlin (Colin Morgan) providing moral support - and how great was that moment early on with their handshake? A rare moment of the King treating his servant as a true equal.
Meanwhile, evil Morgana's busy sassing her way through her latest fiendish scheme - a trap for Arthur, with Gwen as the bait. This may all feel a little familiar, but there's a twist to the witch's latest dose of villainy - it involves the Dark Tower, a foreboding residence where Gwen is kept captive...
The tower is full of things that go bump in the night - you can imagine any younger viewers jumping out of their skin at certain points this week. We hear a lot about how much "darker" Merlin has become series by series, and this week it becomes clear that the show becoming more adult doesn't just apply to emotional maturity...
Clearly those behind the show are now willing to engage in some proper psychological scares and grotesquery too! Angel Coulby's terrific as Gwen suffers something of a nervous collapse, broken down by Morgana's cruel trickery...
Leon and Percival's dark dreams eventually lead Arthur and his men to the equally dark tower - which unfortunately lies beyond an impenetrable forest, called the Impenetrable Forest. No, really.
Merlin ultimately uses his magic to lead Arthur and company out of the woods - too often our hero's great abilities are reduced to 'the power to throw an enemy against a wall', so our his Thundercats-esque 'sight beyond sight' glimpsed here is a fun and novel invention.
The episode culminates in an Indiana Jones-style incursion into the Dark Tower and, in the final scenes, the surprise death of Elyan. He was always one of Arthur's less well-utilised knights, but writing the character out - even in such a noble fashion - is still a bold move. Credit to the Merlin writers and again to Adetomiwa Edun - it's his best performance in the role by a mile.
It's just a shame that Elyan's departure was signposted - the brief appearance of flirty imp Queen Mab was a rather random addition to the episode, and imagine how much more shocking his exit would have been if we hadn't been expecting one of Arthur's knights to depart this mortal coil.
But we'll forgive this episode any slip-ups for that genuinely surprising final twist - Morgana's scheme has worked and Gwen has turned to the dark side. It's a game-changer for sure, though whether or not she's really lost it, or is just the subject of dark magic, remains to be seen - we're betting the latter.
Darker than Merlin's ever dared to be before, 'The Dark Tower' provides something different than the usual derring-do - this show often comes under fire for continuing to exploit an already well-trodden formula, but this week Merlin heads out of its comfort zone a little, with terrific results.