Season 5. Morgana after she became evil. Morgana then used magic on Gwen, turning her evil temporarily. She brainwashed her by trapping her in a tower with mandrakes which made her hallucinate that all those she loved were against her.

How did Morgana turn evil?

Originally a kind-hearted individual, Morgana became evil after being corrupted by Morgause and being betrayed by her former-friend Merlin. ... Soon after the Battle of Camlann, and Mordred's death, Morgana was killed by her nemesis Merlin with Excalibur.

How did Merlin betray Morgana?

8 Merlin: Poisoned Her

One of the worst betrayals Merlin ever committed was when he poisoned Morgana. ... This leads to the ultimate betrayal being committed, with Merlin taking advantage of Morgana's disheveled mind to slip poison into her water.