Workmen unearth an ancient tomb beneath Camelot filled with treasure and strange carvings of ravens. A worker reaches out to touch a magnificent, glowing jewel on the sarcophagus and drops down dead. When Gaius and Merlin investigate, it's only Merlin's magic that prevents Gaius from being killed by the same trap.
Gaius begs Uther to seal up the tomb, fearing it belongs to Cornelius Sigan, a terrifying sorcerer from local legend. The all-powerful Sigan cursed Camelot as he faced death, swearing he would return and raze the city to the ground. But Uther is too taken with the treasure to indulge Gaius, insisting Sigan is just a fairytale.
One of the workmen is approached by a conman, Cedric, who tricks him into revealing that Arthur holds the only key to the tomb. Cedric goes about gaining Arthur's trust. When a huge wild boar charges straight for Arthur while on a hunt, Merlin uses magic to fell the beast just in time, but Cedric takes the credit. Arthur offers him a reward and Cedric chooses a role in the Royal
Merlin, meanwhile, is relegated to mucking out the horses' stables, where Cedric smoke bombs him with a sleeping draft. Merlin passes out in a pile of manure and when he wakes the horses have been released. Arthur blasts Merlin for his recent poor performance and Merlin loses his cool, calling Arthur gullible. Seething, Arthur orders Merlin to think about whether he really wants to be his servant anymore.
Gutted, Merlin tells Gaius that, just for once, he'd like Arthur to see him for who he really is. Gaius urges patience. The prince is in more danger than ever. Gaius has translated the runes found on the jewel in the tomb: "He who breaks my heart, completes my work". The stone contains Sigan's soul. If removed from its setting, the "heart" will be broken and the soul released.
Gaius and Merlin assume the jewel is safely locked away but, as they sleep, Cedric finally gets his hands on Arthur's keys. The moment Cedric frees the jewel from its setting a vapour consumes him and he becomes Sigan. The break-in is discovered and Merlin tries to warn Arthur that Cedric has been possessed, but the prince won't buy it. Arthur tells Merlin his services are no longer required, and calls on Cedric to escort him from the palace. Merlin takes one look at Cedric's eyes and leaps on him. They fight. Arthur steps in and throws Merlin into prison.
As night falls, Cedric/Sigan stands on the roof of the castle and utters a terrible spell, calling on the very walls of Camelot to help inflict his vengeance. A gargoyle begins to stir, then another. Stone eyes flick open and the gargoyles peel away from the walls. In the lower town, people flee as one of the screaming winged monkeys scythes through market stalls. Another gargoyle swoops on Gwen. Arthur gathers his knights, but it's too late - the castle walls start to crack and the palace square has become a battleground.
Arthur takes a hit, losing his sword. The monster swoops in to finish the job, but Gwen pulls Arthur to safety just in time. A moment of frisson between the two of them is cut short as Arthur winces in pain - he's been wounded.
In his prison cell, Merlin can hear the mayhem and knows Arthur's in mortal peril. He utters a spell and the cell door explodes from its hinges. Gaius surprises Merlin by urging him to consult the only one old enough to know Sigan - the Great Dragon. Merlin doesn't think the Dragon can be trusted any more, but Gaius warns that the creature's wisdom should not to be lightly rejected, especially at a time like this.
At first, the Dragon refuses Merlin's request, but Merlin argues that Arthur's prophesied "destiny" will not come to pass if Sigan triumphs. The Dragon agrees to give Merlin a spell of the Old Religion to vanquish Sigan, but with no guarantee it will work and only on the condition that Merlin gives him a promise that, one day, Merlin will set him free.
Ordering his knights to lock the doors of the palace and save the King, Arthur makes a last stand alone. The gargoyles attack and he is knocked down, semi-conscious, but Merlin arrives, sending a powerful flash of magic through the gloom. The beast returns to stone mid-flight. It plummets, smashing to pieces. Merlin rushes to Arthur.
Cedric/Sigan is impressed by Merlin's power. He questions Merlin's loyalty to one who treats him like a slave. Sigan offers to help Merlin unleash his true power. The world will finally appreciate his greatness and Arthur will see him for what he truly is. Merlin is tempted. But when Sigan talks of the two of them ruling together and teaching Arthur to fear Merlin, Merlin balks. Better to serve a good man than rule with an evil one.
Sigan seethes. If Merlin won't join him, he will simply possess Merlin and harness his power. A terrified Merlin acts quickly, uttering the Dragon's spell as vapour surges across the ground and coils its way up his legs. His body jerks violently. His eyes flick black and he lies still.
Gaius searches amidst the devastation. Merlin approaches, but there's something cold about his eyes. For an awful moment, it seems Merlin has been possessed, but then he breaks into a grin and holds up the jewel: it once again glows with life. Gaius and Merlin embrace.
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