What is the name of the villain in 'The Sword in the Stone'?

The villain in “The Sword in the Stone” is Madam Mim.

Madam Mim is an eccentric, black-magic-practicing witch and Merlin’s longtime nemesis in Disney’s 1963 animated film. Arthur, still known as Wart, encounters her while Merlin is away. Mim quickly reveals her hostility toward Merlin and her delight in using magic to cause trouble.

Her main story is the wizard’s duel with Merlin. The pair transform into a succession of animals while trying to outwit one another, and Mim repeatedly cheats or ignores the duel’s rules. Merlin ultimately defeats her by transforming into a germ and making her ill, turning the contest into a comic lesson about intelligence overcoming brute force.

Madam Mim is sometimes overlooked because she appears in only one major episode and is not responsible for Arthur’s larger political conflict. Merlin is Arthur’s mentor, Archimedes is his scholarly owl, and Wart is Arthur’s childhood name—not the villain. Mim’s role is that of the film’s principal magical antagonist, even though the movie is structured more like a series of lessons than a continuous villain plot.