 The primary sources for the "empousa" in Antiquity are Aristophanes's plays ("Frogs" and "Ecclesiazusae") and Philostratus's "Life of Apollonius of Tyana".The Empusa has been defined in the Sudas and by Crates of Mallus as a "demonic phantom" with shape-shifting abilities.Thus in Aristophane's plays she is said to change appearance from various beasts to a woman.