Empusa or Empousa (Ancient Greek: Ἔμπουσα; plural: Ἔμπουσαι Empousai) is a shape-shifting female being in Greek mythology, said to possess a single leg of copper, commanded by Hecate, whose precise nature is obscure. In Late Antiquity, the empousai has been described as a category of phantoms or spectres, equated with the "lamiai and mormolykeia, thought to seduce and feed on young men.
The Empusa has been defined in the Sudas and by Crates of Mallus as a "demonic phantom"[a] with shape-shifting abilities. Thus in Aristophane's plays she is said to change appearance from various beasts to a woman.
The Empusa is also said to be one-legged, namely, having one brass leg,[b] or a donkey's leg, thus being known by the alias Onokole. A folk etymology construes the name to mean "one-footed" (from Greek *έμπούς, *empous: en-, one + pous, foot).
In Aristophanes's comedy The Frogs, an Empusa appears before Dionysus and his slave Xanthias on their way to the underworld, although this may be the slave's practical joke to frighten his master. Xanthius thus sees (or pretends to see) the empousa transform into a bull, a mule, a beautiful woman, and a dog. The slave also reassures that the being indeed had one brass (copper) leg, and another leg of cow dung[c] besides.
The Empusa was a being sent by Hecate (as one scholiast noted), or, was Hecate herself, according to a fragment of Aristophanes's lost play Tagenistae ("Men of the Frying-pan"), as preserved in the Venetus.[d]
By the Late Antiquity in Greece, this became a category of beings, designated as empusai (Lat. empusae) in the plural. It came to be believed that the spectre preyed on young men for seduction and for food.
According to 1st century Life of Apollonius of Tyana, the empousa is a phantom (phasma) that took on the appearance of an attractive woman and seduced a young philosophy student in order to eventually devour him. In a different passage of the same work, when Apollonius was journeying from Persia to India, he encountered an empousa, hurling insults at it, coaxing his fellow travellers to join him, whereby it ran and hid, uttering high-pitched screams.
An empousa was also known to others as lamia or mormolyke. This empousa confessed it was fattening up the student she targeted to feed on him, and that she especially craved young men for the freshness and purity of their blood, prompting an interpretation as blood-sucking vampire by Smith′s Dictionary of Greek and Roman Biography and Mythology (1849).
In modern times, folklore has been collected about a being fitting the description of an empousa: an extremely slender woman with multiple feet, "one of bronze, one a donkey's foot, one an ox's, one a goat's, and one human", but she was referred to as a woman with the lamia-like body and gait. The example was from Arachova (Parnassus) and published by Bernhard Schmidt (1871) Schmidt only speculated that oral lore of empousa might survive somewhere locally. A field study (Charles Stewart, 1985) finds that empousa is a term that is rarely used in oral tradition, compared to other terms such as gello which has a similar meaning.
According to Robert Graves, Empusa was a demigoddess, the beautiful daughter of the goddess Hecate and the spirit Mormo. She feasted on blood by seducing young men as they slept (see sleep paralysis), before drinking their blood and eating their flesh. When she spotted a man sleeping on the road, she attacked him, little knowing he was Zeus, king of the gods. Zeus woke and visited his wrath on her and Empusa was killed. 
Empusa is a character in Faust: Part Two by Goethe. She appears during the Classical Walpurgis Night as Mephisto is being lured by the Lamiae. She refers to herself as cousin to Mephisto because she has a donkey's foot and he has a horse's.
Empusa is a main antagonist turned heroine in the novel Grecian Rune by James Matthew Byers. They may look like humans at first.
Empusa (along with Lamia and Mormo) is one of the three witches in the film Stardust (dir. Matthew Vaughn). She is played by Sarah Alexander. In Neil Gaiman's novel Stardust the witches are not given individual names.
έχει κορμί τής Λάμνιας ή πώς περβατεί σαν τη Λάμνια.. mehr als zwei und zwar verschiedenartig gebildete Füsse hat, der eine ist von Erz, der andere ist ein Eselsfuss, wieder ein anderer ein Ochsenfuss, ein Ziegenfuss, ein Menschenfuss u. s. w.