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?