Femme Fatale

A Femme fatale, is a beautiful female often times associated with a Villainess that seduces the Hero or Heroine, helps them out or often times leads them to their downfall/ trap. An example of a femme fatale would be the Sirens in Greek mythology who had beautiful singing voices that caused male sailors to seek them out and their ships to crash into the sea rocks.

See Also:

Villainess

Femme Fatale article on wikipedia