Merlin is to face his toughest challenge yet when Morgana's blind determination puts at risk Arthur's future as well as the very balance of the world. With her unbounded powers at their highest, the sorceress uses powerful magic to tear the mystical veil between worlds making the mighty Cailleach to come forth. Hellish creatures are released and wander around, killing everyone who succumbs to their touch. With Uther but a mere shadow of his former self, it comes down to Arthur and his loyal knights to safeguard the kingdom. However, swords won't be enough to vanquish the newest threat and Merlin is utterly shocked to find out what it'll take to restore the equilibrium of the world: a sacrifice of unimaginable proportions is required.

The kingdom is on the verge of complete collapse and Morgana is poised to strike. Lancelot races back to Camelot with the dying Merlin leaving Arthur to continue on his mission to vanquish the Dorocha alone. Friendship and loyalties end up being tested to the limit. It is the power of love that ends up changing all their lives for ever though.

In this episode, The Prince Arthur's birthday is coming near. All the pwople of the kingdom are humming about celebrations of Arthur's birthday. Odin, who is Arthur's old antagonist, make him sure that there will be no problm on that special day. But a ruthless bravo gas planned to attack the Prince.

In this episode 04 of Merlin Season 04, Julius Borden turns out to Camelot as soon as he comes to know about the changing of the Camelot's Kingdom landscape because of some secret magical powers. He wants to reach the tomb of Ashkanar in order to get the last egg of dragon. Merlin is very happy on his arrival.

In this episode 05 of Merlin Season 04, Arthur realizes the responsibilities of the king and he makes his mind to prove himself a good leader. He also faces the anger of his Queen Annie. Arthur has to make correct and immediate decisions for the benefit of his realm. He search for his inner strength and become self dependent.

Merlin is now under the control of Lady Morgana. She makes Merlin her main weapon in order to dominate over the kingdom of Camelot. No one is aware about this. Morgana makes so many misunderstandings between Merlin and the King Arthur. The king is not at all aware about the danger he is going to face.

In this episode 07 of Merlin Season 04, Morgana unites with the ruthless wizard, Alator. Their plan is to reveal a secret with which their fate is changed. The castle is surrounded by Morgana and Alator and they are to be stopped by someone otherwise the Merlin's secret will be exposed to everybody.

In this episode 08 of Merlin Season 04, Merlin goes to a village, where some disease strikes, in order to heal it. Arthur and Gaius find that a girl, Lamia, is responsible for this cause. They soon out ride to the village to help Merlin and Guinevere as they are not aware about Lamia. The knights also suffer from that illness.

In this installment, Sir Lancelot comes back to the Kingdom of Camelot and all the people are surprised to see him. Merlin has doubt on Lancelot and tries to find the truth behind his returns as Lancelot is thought to be dead. To save Camelot, Gwen is controlling over her enticement so that she could not perform any disloyalty.

In this episode 10 of Merlin Season 04, the knights of Camelot find a supernatural place when they are going through the forest. It is good for all not to create any disturbance there. Merlin admonishes Elyan but he does not care about that. Elyan later on suffers so many problems that nobody can even imagine.

In this installment, Lady Morgana has found a new friend who is a military leader named Helois. They has united and decided to defeat the kingdom of Camelot and take over the rule. The King Arthur is confined in thoughts of Queen Mithian. Arthur is not missing his friend who always help him in his problems.

Lady Morgana is now ruling over a large army of Southron. The kingdom is enclosed by a mortal net. Morgana has magical powers and she attacks the kingdom. All the people are running here and there to save their lives. The King Arthur is also in danger. Now only Merlin can save the life of Arthur from Morgana.

Morgana and Helios, after so many difficulties, are successful in taking the rule over the kingdom of Camelot. Merlin and the King Arthur and his other friends are escaping from them. Arthur feels that he is not a good leader but Merlin makes him sure that he should react bravely and then they again attack Morgana. This time the victory is in the hands of Arthur.