In Greek lore, a malevolent and sensual female demon. They could assume the form of either animals, or that of beautiful women, and when they were in the form of women, they behaved as succubi and copulated with men.
They were the daughters of Hecate, and scared to death or ate anyone who traveled along the paths and roads that they inhibited. They are sometimes described as having donkey-like legs and brazen slippers.
One way to get rid of the empusae is to insult them, which will cause them to run off screaming. (from Mystical Creature A Day)