Empusa (Ancient Greek: Ἔμπουσα, Empousā, of unknown meaning) is a demigoddess of Greek mythology. In later incarnations she appeared as a species of monsters commanded by Hecate (known in English as an empuse).
As a demigoddess 
Empusa was 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. Empusa is pictured as wearing brazen slippers and bearing flaming hair. By folk etymology, her name was said to mean "one-footed" (from Greek *έμπούς, *empous: en-, one + pous, foot). This gave rise to the iconography of a one-legged hybrid, with a donkey's leg and a brass prosthetic leg.
Later and modern usage 
In later Greek mythology, her role was reduced to a species of spectre called an empuse or empusa (pl. empusae). The empusae were sent by Hecate to guard roads and devour travelers (Hecate was also the goddess of roadsides). According to Philostratus, empusae ran and hid, uttering a high-pitched scream, at the sound of insults. The empusae are best known for their appearance in Aristophanes's The Frogs, in which they scared Dionysus and Xanthias on their way to the underworld.
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.
In the Percy Jackson & the Olympians series, they first appear in The Battle of the Labyrinth as servants of Hecate, who had by that time joined the Titan Army. The first empousa, who claimed to be freed from Pandora's jar when it had been opened, appeared in the next book, The Last Olympian.
In the British webseries 'I Am Tim', the Empusa is a female demon who is defeated by the hero, Tim Helsing in Series 1, episode 2. The demon has the ability to turn into fire at will and is eventually defeated by using an amulet and saying sacred words which a character "looks up on wikipedia" on his iphone. 
The Empusa is an enemy in the video game God of War: Ascension.
- An Intermediate Greek-English Lexicon, Liddell and Scott
- "Empuse" at Dictionary.com
- "Empuse" in Webtser's Revised Unabridged Dictionary (1913)
- "Empuse" in Webster's Dictionary (1928)
- Graves, Robert (1990) . "The Empusae". The Greek Myths. London: Penguin. pp. 189–90. ISBN 0-14-001026-2.
- "Empousai" on Theoi Greek Mythology
- "Empusa" in the Greek Myth Index
- Matthews, John and Caitlin (2006) . "Empusa". The Element Encyclopedia of Magical Creatures. New York: Barnes & Noble. p. 202. ISBN 978-0-7607-7885-2.