An ancient tomb was discovered by miners beneath Camelot, filled with treasure. Prince Arthur Pendragon sent his servant Merlin to resolve the noise issue which resulted from the excavation (and keeping Arthur from sleeping). Merlin witnessed panic as the workers fled the site when one of them was killed by a booby trap. Gaius studied the tomb, in particular a large heart-shaped jewel set in a sarcophagus at the tomb's centre, a thief named Cedric learned of the jewel and worked his way into the castle in order to gain access to the tomb, granting him entry to the treasure. By muscling in on Merlin's position as Arthur's manservant, Cedric successfully turned Arthur against Merlin, leaving Merlin once more annoyed and depressed that the prince couldn't see him for who he really is.Gaius realised that the tomb belonged to an evil sorcerer named Cornelius Sigan, who had sworn to destroy Camelot upon his death. Sigan, fearful that his power and wealth would die with him, found a way to make himself immortal: he stored his soul inside the large jewel, which would then possess the person who removed the jewel from its setting. Gaius' warning proved unfruitful when presented to Uther, who was too taken with the treasure and too proud to believe in such a myth. The next night, Morgana, a Seer, was awakened by nightmares featuring a raven, which solidified Gaius's theory of the return of Sigan, as the raven was an emblem of his power.
Once Cedric had stolen Arthur's keys to the tomb, he came across the heart-shaped jewel containing Sigan's soul. By removing it from its setting, Sigan's soul was released and possessed Cedric's body. Gaius and Merlin realise the consequences of the jewel being removed the next day and Merlin deducted that the only other person with access to the key would be Cedric, and ran to alert Arthur. Arthur refused to believe Merlin and after a scuffle with Cedric, Merlin was taken to the cells.
That night, Sigan invoked a spell which animated the gargoyles positioned around the castle in his attempt to destroy Camelot. As Merlin remained locked in his cell, the creatures caused havoc in the city, killing many citizens. Arthur and his soldiers faced off against them, many of them perishing. Gaius, Gwen and Morgana tend to the wounded inside the castle while Merlin used magic to break out of his cell. As Arthur faced one of the creatures alone in the square, Gwen saved his life twice before taking him inside to tend to his injuries. Arthur nervously complimented Gwen on her bravery before thanking her as she left to prepare a dressing for his wound. Gaius informed Merlin that the only way he can stop Cedric is to consult the Great Dragon, to which Merlin was initially opposed to. He relented however, and rushed to ask the Dragon for advice. The Great Dragon made Merlin promise to release him one day in return for a powerful spell which would defeat Cedric, something Merlin reluctantly agreed to.
Meanwhile, Arthur refused Uther's proposal to shut off the citadel to protect those with a chance of surviving and rallied his men to leave the castle and face the creatures once more, as it is his duty to Camelot and to himself. Arthur and his men were pushed back inside the square by the creatures and he ordered the soldiers to retreat, as he faced the creature on his own. As Arthur's men retreated inside the castle, Arthur was knocked unconscious by the creature. Before it can kill him, Merlin arrived and destroyed the gargoyle with his magic.
Merlin arrived to find Arthur's body and encountered Sigan, who tried to entice Merlin to join him in ruling the land with their magic. Merlin was perplexed by the proposal as he was still affected by the lack of respect he received from Arthur, but felt it was still better to serve a good man than to rule alongside an evil one. The soul of Sigan left Cedric's body to try and possess Merlin's in order to conjoin both magics into one superpower. Merlin recited the spell which forced Sigan's soul back into the jewel, bringing safety upon Camelot once more.
The tomb of Sigan was covered up once more, with the jewel inside which was to never be removed again. After this attack, Uther was more passionate than ever to eradicate all form of magic from the kingdom while Arthur, in his own way, apologised to Merlin by making him clean his armour (The Curse of Cornelius Sigan).