The Lady of the Green Kirtle, also known as the Queen of Underland, the Witch-Queen, and the Emerald Witch, is the main antagonist in C.S. Lewis's Narnia book, The Silver Chair, as well as the BBC movie based on it where she is portrayed by Barbara Kellerman (who also played Jadis the White Witch and the Hag). Her real name is unknown, but it is speculated that she is a descendant of Jadis the White Witch, and thus has giant blood running through her veins.
The Emerald Witch had the ability to transform herself into a large, venomous green snake. In her plan of the conquest of Narnia, she used this form to murder the Queen of Narnia, King Caspian's wife. Her son by the name of Prince Rillian then set out to avenge his mother's death, but was captured by the Emerald Witch and held as a prisoner in her underground lair. She seduced and brainwashed him into forgetting who he was and that together they would conquer Narnia. Her spell, however, was taken away for one hour each day, and Rillian would once again remember who he was, so during that hour, she would have Rillian bind himself to a silver chair until he once again forgot who he was.
When the children Eustace and Jill with their companion, Puddleglum the Marshwiggle, came to Underland to free Rillian, she tried and nearly succeeded in putting them under a hypnotic spell in which they forgot all about Narnia, the sun, and Aslan. Puddleglum, however, broke free from the spell by sheer willpower and stamped out the intoxicating green flames which the Witch had used, along with music, to hypnotize them. The Witch then transformed herself into the deadly green snake again and attacked Rillian and the group, and, after a hard battle, they succeeded in killing her.
In the books, the gnome-folk which she had ruled over thanked the heroes for defeating her. In the BBC / Wonderworks film, however, most of them are shown throwing themselves into a fiery abyss upon her defeat.
The Horse and his Boy