01E07 - The Long Night

Despite the misgivings of Ygrain and Kay, Arthur and his inner circle attend a banquet given by Morgan as a sign of reconciliation, during which her castle is attacked. Arthur's men prepare to defend it but Merlin is suspicious of Sybil as the 'attack' is evidently stage-managed by Morgan to break up the party. Merlin tells Ygrain Morgan killed Pendragon but asks her to keep the news from Arthur. Morgan overhears Arthur tell Guinevere he loves her, which she sees as her key to destroying him. After the 'enemy' has been repulsed Arthur and his party return to Camelot but Morgan has used magic to turn herself into Ygrain, keeping the genuine queen a prisoner in her castle.