At Arthur's coming of age ceremony, the celebrations are suddenly interrupted when a mysterious Black Knight crashes his horse in through the window. He throws down his gauntlet, challenging the knights of Camelot to single combat. A young, eager Knight, Sir Owain, picks it up, accepting the challenge. Uther is white as a sheet as he watches the knight go, staring intently at the distinctive crest which marks his helmet.
The following day a large and nervous crowd gathers to watch the fight. Sir Owain strides out to the waiting Black Knight and the pair do battle. The Black Knight is an awesome warrior, and it looks like Sir Owain doesn't stand a chance. However, Merlin is delighted when the young Knight successfully strikes his opponent...but the Black Knight doesn't even flinch and, seemingly unaffected by the blow, he goes on to slay Sir Owain.
The Black Knight steps over the body on the ground and throws down his gauntlet again. Arthur makes to step forward but is stopped by Uther. Before he can react, Sir Pellinor steps forward and accepts the challenge.
Gaius takes Merlin down into the crypt below Camelot and to the tomb of Tristan De Bois. Gaius is horrified to see that it is empty - his body has gone. The stunned Merlin listens as Gaius explains that the mysterious Knight bears the crest of Tristan De Bois, who was the brother of Uther's wife. Driven mad with grief, he blamed Uther for Ygraine's death in childbirth. Gaius tells Merlin that he thinks the Knight is Tristan De Bois, brought back from the dead to exact his revenge.
The next day, Sir Pellinor fights the Black Knight. Once again, the Knight kills his opponent. However before the Black Knight can throw down his gauntlet, Arthur pre-empts him - the next fight will be between the two of them. Uther is furious with his son's actions, but Arthur is adamant.
Merlin and Gaius are now convinced that the Knight is indeed Tristan. Merlin feels compelled to find a way for Arthur to defeat him. Merlin discovers that there is only one way to kill a vengeance wraith - with a sword forged in a dragon's breath. Merlin goes to the Great Dragon and asks if he will do it. The Dragon refuses, but Merlin implores him - Arthur needs the sword, otherwise he will die. The dragon relents - he will do it for the young Pendragon, for their destinies are entwined, but Merlin must promise not to give the sword to anyone else.
Meanwhile, Uther is visited by Nimueh. The evil witch admits that it was her who raised Tristan from his tomb. She felt it was time Uther faced up to the past. We discover that Nimueh played an integral part in Arthur's birth, helping Ygraine conceive with magic...but that the price of providing Uther with an heir was his beloved Ygraine's life. Nimueh delights in the situation she has created. The Black Knight will exact the revenge so many seek, not stopping until Uther is dead. Uther will watch Arthur die at the hands of the wraith and she will enjoy every moment of his suffering knowing that he'll be next.
Uther comes to Gaius. He admits that he knows who the Knight is; Tristan has returned from the grave to punish him - a punishment which he thoroughly deserves. He has to choose between his own life and his son's; they both know that there is only one thing he can do. Gaius realizes that Uther intends to fight the Black Knight in Arthur's place. He wants the Physician to help him.
That night, Gaius takes Arthur a potion to make him sleep. He locks the door, trapping Arthur in his room.
The following morning Merlin prepares Arthur's armour and the new sword. But it is not Arthur who enters the armoury. It is Uther. He asks Merlin, as Arthur's servant, to prepare him for the battle. Merlin is wary but has no choice, he has to give him the sword - the sword which he has readied for Arthur's use...the sword he promised he would not give to anyone but the Prince.
The Black Knight and Uther have an epic fight. Shock turns to horror when Uther rips off the Black Knight's helmet to reveal a hideous wraith underneath. The new sword strikes home and the wraith explodes in a cloud of dust. Uther looks down at the wraith's remains, astonished, as Arthur comes running out. Merlin visits the Great Dragon who is furious with him; he tells Merlin to dispose of the sword where no-one will ever find it. Merlin takes it to a lake and throws it into the water, oblivious to the fact that Nimueh is watching, plotting her next attack.
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