The title is yet to be confirmed by the BBC but it will be the first instalment of a two-parter believed to be called The Diamond Of The Day, which will end the show's final series.
It guest-stars Peter Guinness as Ari and is currently scheduled to air on BBC One at 7.55pm on Saturday 22nd December.
High in the mountains a great horde gathers. Morgana prepares for war, with a vengeful Mordred at her side. Merlin feels the weight of his destiny like never before as the ancient prophecies play out with terrifying accuracy. But before he can save his beloved Camelot, he must save himself. For it is not just the kingdom Morgana wishes to destroy – it is also Emrys.
As previously reported, pictures from episode 11 will be released tomorrow. Images from the show's final episode were released last month.
UPDATE - 4.25pm: The final episode of Merlin will air on BBC One on Monday 24th December at 8.15pm, it was announced today.