Lady of the Green Kirtle

The Lady of the Green Kirtle, also called the Queen of Underland, the Witch-Queen, or (to differ her from the White Witch) the Green Witch, was a beautiful but villainous sorceress who ruled  Underland, below Narnia.

The Fall of Underland
See the main article, Fall of Underland

Her origins are extremely mysterious, though it is known she had the extraordinary ability to transform into a great green serpent. She began to initiate a plot that would capture Narnia which involved the abduction of Prince Rilian, son of Narnia's king Caspian X, and also resulted in the death of Rilian's mother, the Narnian queen, in Narnian-year 2345.

The Green Witch, with the motive to take over Narnia with an army of Earthmen and rule behind a puppet King Rilian, kept Prince Rilian in captivity for several years, imprisoning him through an ongoing enchantment that she regularly renewed using a magical device called the silver chair. When Jill Pole, Eustace Scrubb, and Puddleglum arrived from the Kingdom of Narnia to rescue the missing prince, Rilian destroyed the chair and, after throwing off the Witch's other enchantments, decapitated her as she attempted to constrict him in her serpentine form.

Her death activated another series of enchantments that caused Underland to collapse upon itself, although Rilian and the others escaped and returned to Narnia.

Trivia
She is also sometimes believed to have been Jadis, the White Witch, in a resurrected state. This theory is probably provoked by the fact that in the BBC television series, Jadis and the Lady of the Green Kirtle are both played by Barbara Kellerman (though this was more to do with budgeting rather than a statement on the nature of the Lady of the Green Kirtle). Further, though she was killed by Aslan, she ate an apple, perhaps allowing her immortality. However, Aslan himself said that Jadis was dead, despite other factors. In the books, the two are explained (as separate individuals) to both be a part of a mysterious group known as the "Northern Witches." This supports the theory that she was a follower of Jadis (with similar goals to Jadis), rather than Jadis herself in another form.