Alto Villainess

A subtrope of Evil Sounds Deep. In musical theatre, opera, etc. the main protagonist will often be tenor if male or soprano if female. If the villain is female, however, she will often sing alto.

Theatre

 * Mrs. Meers from Thoroughly Modern Millie.
 * Mrs. Lovett, one of the two Villain Protagonists in Sweeney Todd: The Demon Barber of Fleet Street.

Western Animation

 * Ursula from The Little Mermaid.

Other Media

 * It's common enough in opera to be a crossword puzzle clue.