第 1 季，第 9 集: A mysterious knight arrives in Camelot, uncovering a secret from the past that Uther would prefer stayed buried. At Arthur's coming-of-age ceremony, the celebrations are interrupted when a mysterious Black Knight crashes his horse through the window. He throws down his gauntlet and challenges the knights of Camelot to single combat. Sir Owain, a young, eager knight, picks it up and accepts the challenge – they will fight the next day. Uther is as 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. He sees the blade go in but the Black Knight doesn't even flinch and, seemingly unaffected by the blow, he goes on to slay Sir Owain. Later, 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. 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 his sister's death in childbirth. Gaius tells Merlin that he thinks the Black Knight is Tristan De Bois, brought back from the dead to exact his revenge. Merlin discovers that there is only one way to kill a vengeance wraith – with a sword forged in a dragon's breath. But is it too late for Arthur who, determined not to lose any more of his knights, is due to fight the Black Knight the following day?