The preview also teases that Arthur will try to pull Excalibur from the stone that has been guarding it.
The girls will take part in a battle when "Merlin" reaches its season finale in the U.K. After Morgana and Helios have successfully captured Camelot, Gwen will face off Morgana in a sword duel which seemingly puts the former in a difficult situation.
Meanwhile, trapped in Ealdor with a ruthless army closing in, Arthur and Merlin's situation becomes desperate. The young king must take control before Camelot is lost forever. But faced with Agravaine's betrayal and the strength of Morgana's force, Arthur has lost his conviction.
In the meantime, only Merlin can restore his faith, and only an extraordinary act can prove to the king that he was born to rule. The said act may include Excalibur since a preview for the episode sees Arthur trying to pull the sword from the stone that has been keeping it.
"The Sword in the Stone (Part 2)", which is the latest outing from "Merlin" season 4, will air Saturday, December 24 on BBC One. In the U.S., the fourth season will premiere January 6, 2012 on Syfy.