A year after her disappearance and when almost all hope is lost, Morgana returns to Camelot. While Merlin is afraid she might remember the circumstances of her disappearance and give him away in front of the king, a delighted Uther announces a banquet to celebrate. However, soon afterwards evil magic starts to take hold of the king's mind, putting the safety of Camelot at risk as he is consumed with hallucinations and enemies reach his door. Merlin suspects Morgana is behind the enchantment but his search for the truth puts his life in peril while Arthur is forced to take control and face new foes on his own. Will they succeed to rescue the king and Camelot from destruction?

With the help of the Great Dragon, Merlin manages to return to Camelot only to find out that is it on the verge of invasion: Morgause and Cenred have teamed up and plot against a weakened Uther. Cenred's initial reluctance is won over by the fact that they count on an ally within Camelot walls who will ensure the castle is taken. Uther is unable to rule which forces Arthur to step up front and defend the citadel agains Cenred's powerful army. With the help of Morgause and to make things worse, Morgana plays a deadly card and summons a skeleton army to wreak havoc within the castle walls. Camelot's fate rests once again on Arthur's shoulders and Merlin's magic as they fight in their own way to hold off both an army of the living and an army of the dead. Will they manage to succeed?

When Merlin comes across a secret chamber, he accidentally releases a goblin. The mischievous little creature possesses Gaius, making the wise and renowned court physician turn into a shadow of his former self, quickly developing a taste for gold, ale and magical jokes. To make things worse, the king suddenly becomes bald and a nasty scourge of flatulence hits the royal household. Will Merlin be able to figure out the source of all troubles, return the gobblin back to where he came from and put an end to the havoc before any real damage is done?

Merlin and Arthur find themselves outnumbered in a bar fight but an enigmatic young man helps them win. The man’s name is Gwaine and he risks his own life to protect Arthur but during his act of bravery he is seriously wounded. The Prince decides to take him back to Camelot where he could make a recovery with Gaius' help. Gwaine immediately gets to work: drinking, flirting with the ladies and causing lots of trouble for Merlin who has been instructed to look after him. However, when two unscrupulous knights seem to plot to murder the Prince, will Merlin and Gwaine be able to stop them and save Arthur?

After an unexpected meeting with the enigmatic old sorcerer Taliesin, the wizard takes Merlin to the crystal cave allowing him to witness a glimpse of the future. However, this gesture turns out to be more of a curse than a gift when, much to his horror, Merlin sees Morgana set to kill Uther. The young warlock knows he must do his best to try and stop the future from playing out. But changing the pages of destiny is a very dangerous game and his actions end up having fatal consequences. With the whole of Camelot in a state of grief, will Merlin be able to change the course of destiny and undo the damage he has caused, or will it be too late?

Uther pressures Arthur into an arranged marriage with a visiting princess, Elena, though Merlin soon has reservations after discovering that Elena may not exactly be who she claims to be. As he tries to uncover the princess' dark secret, he learns that Elena's nanny is a Pixie. But what are their motivations? Who do they work for? And most important of all, what benefit do they get from her marriage to Arthur, the heir to the throne of Camelot? Will Merlin be able to get to the bottom of it all and prevent Arthur from making the biggest mistake of his life?

Gwen faces an impossible dilemma when she is forced to choose between saving the life of Elyan, her estranged brother and only sibling or Arthur's, Camelot's prince and the love of her life. When one of them is certain to find death at the hands of Cenred, who will Gwen choose?

Once again, loyalty, love and friendships are tested to the limit as Arthur, Merlin and Gwen are forced to put their minds together to try and find a way out of the cruel quandary. Morgana and Morgause, in turn, will do their best ruin their plans.

Will good conquer evil or will Gwen end up losing someone she loves again?

Arthur embarks alone on a quest to prove himself worthy of the Camelot throne by retrieving the Golden Trident from the Fisher King.

With the Prince beyond the protection of the citadel, Morgana takes the opportunity to use her dark magic. She gives him a precious bracelet that contains a Phoenix Eye, insisting that he wears it at all times for protection. However, the magical jewel takes away Arthur's life force and he is left defenceless in the Fisher King's perilous realm.

With the help of Gwaine, will Merlin be able to reach Arthur in time and fulfil his own quest to protect the future king of Camelot?

An old flame of Gaius' arrives in Camelot under the influence of a Manticore.

When Uther banishes Gwen from the kingdom, Arthur resolves to leave Camelot with her.

Ένα ακόμα πρωτάθλημα ξεκινά και το Κάμελοτ έχει γεμίσει με τρομακτικούς πολεμιστές, αλλά και έναν μικρόσωμο άντρα με ένα μυστικό όπλο. Εντωμεταξύ, η Μοργκάνα παραπλανεί τον Αρθούρο και τον Ούθερ ώστε να συμμετάσχουν στους αγώνες.

Ο Αρθούρος και ο Μέρλιν αναλαμβάνουν να ανακτήσουν από τους δρυΐδες ένα κύπελλο που χαρίζει την αθανασία σε όποιον πιει από αυτό. Όμως, όταν αυτό πέφτει στα χέρια της Μοργκός, οι συνέπειες για το βασίλειο είναι καταστροφικές.

Φινάλε 3ου κύκλου. Με τον αθάνατο στρατό της Μοργκός, η Μοργκάνα έχει πια τον απόλυτο έλεγχο του Κάμελοτ, ενώ ο Ούθερ παρακολουθεί από το μπουντρούμι του. Τώρα, μόνο οι ήρωές μας μπορούν να σώσουν το Κάμελοτ από τη βασίλισσά του.