Arthur's one-time fiancée Princess Mithian comes in search of the King's help, but only Merlin and Gwen seem to see the earmarks of a trap in "Another's Sorrow", an all-new episode of Merlin premiering Friday, Jan. 25, at 10 p.m. ET/PT Friday on Syfy.

Reprising her Season 4 role as Princess Mithian is Janet Montgomery, most recently the star of the CBS series Made in Jersey. Montgomery's film credits include the award-winning Black Swan and Our Idiot Brother, and she has had recurring roles on the popular television series Entourage and Human Target.

In "Another's Sorrow", Morgana and King Odin make a sinister pact to lure Arthur to his death - using Princess Mithian as the perfect bait. To ensure the task is completed, Morgana conjures a deception so powerful that the whole of Camelot is taken in - even Merlin himself. But as the storm clouds gather, will the young warlock see through the lies... or will Morgana at long last fulfill her dreams of revenge?

Merlin fans have become accustomed to lead actor Colin Morgan's transformations to his elder version, Emrys. This Friday night, it's Katie McGrath's turn in a prosthetic mask as Morgana disguises herself to move freely about Camelot. Here's what the actress had to say about putting on the prosthetics for the first time.

"Everybody says 'You got to do prosthetics' like it's the most amazing thing - but I have to admit that I was massively allergic, and it was very painful. Colin is so good at it, and they've done it for so many years with Colin, that I think they just expected they'd put it on me and it would all be fine. Instead, I got very sick and it really was not pleasant. Then everybody was wonderful about it, making sure I wasn't in the prosthetic for too long and that I got resting periods."

McGrath said that, as she never quite understood the degree of difficulty to act behind a false face, she developed even greater respect for her dear friend Morgan's acting ability.

"What you don't realize is that acting in a prosthetic really does sap your energy. You're exhausted by it. You're constantly aware of something on your face. Colin was really good about telling me how it works and how the subtle movements on your face don't translate, so you almost have to over-do everything because you're moving your face and then that's moving something on top. Small movements don't work, so it's a completely different way of acting."

And what did McGrath think of her appearance?

"I actually resembled my grandmother, which was really weird."

Merlin fans will be able to see the Morgana transformation for themselves when "Another's Sorrow" debuts Friday, Jan. 25, at 10 p.m. ET/PT, only on Syfy.

Cinemark Movie Club