The empousai are the beautiful daughters of the goddess Hecate and the spirit Mormo. They feast on the blood of seduced men as they slept. Empousai are created by combining together dark magic, animal, and bronze.
Empousai are pictured as wearing brazen slippers and bearing flaming hair. The name is said to mean "one-footed." This is because they are a one-legged hybrid, with a donkey's leg and a Celestial Bronze prosthetic leg.
There is species of Hecatean demon called an empuse or empousa. The empusai are sent by Hecate to guard roads and devour travelers. According to some records, the empusai ran and hid, uttering a high-pitched scream at the sound of insults.
The empousai are best known for their appearance in Aristophanes's The Frogs, in which they scared Dionysus and Xanthias on their way to the Underworld. They originated when Pandora opened the pithos that she was given. When she opened it, the first empousa was released.
There are two empousai masquerading as cheerleaders named Tammi and Kelli at Goode High School. Percy manages to kill Tammi with his celestial bronze sword, Riptide, with the assistance of Rachel Dare, a mortal who can see through the Mist. Kelli, however, escapes by bursting into flames. The heroes later encounter Kelli in Daedalus' workshop. She brings in the ghost of Minos, Daedalus' arch enemy, Nico di Angelo, and a few other monsters. She corners Percy after his sword is knocked out of his hand, but she is soon stabbed in the back by Annabeth Chase.
An empousa holds Prometheus' arm as they approach Percy under a flag of truce. The empousa mentions that she was one of the spirits released when Pandora opened the pithos containing demons and disease. She later bares her teeth at a Hyperborean giant.
When walking through Tartarus, Annabeth Chase and Percy Jackson spot five empousai making their way to the Doors of Death. Among the five were Kelli and Serephone. Annabeth tricks Kelli by talking her into killing Serephone. As the empousai are about to kill Percy and Annabeth, Bob the Titan comes and saves them, killing the empousai with his broom.
The empousai reappear when Frank Zhang, Hazel Levesque, Jason Grace, Leo Valdez, Nico di Angelo, and Piper McLean are making their way toward the mortal side of the Doors of Death. They are among the countless monsters that attack Frank, Jason, Nico, and Piper. The empousai use their charmspeak to command the other monsters, until Piper is able to use her charmspeak to cause the empousai to fight one another to the point that they can no longer give commands, allowing the four demigods to destroy them and their army.
Empousai are shown to be extremely beautiful and seductive women. However, an empouai's true form is a woman with flaming hair, white skin, red eyes, fangs, and one prosthetic bronze leg and one donkey leg. This form is used to drink the blood of the men they enchant.
Empousai are known to prey on young men to feast on their blood. With their stunning looks, they can make a man fall under their spell to incapacitate them. They can also offer a kiss to the young man in hopes of having him come close to her so that she can prey on them easily. In some cases, if the man has enough willpower, like Percy Jackson and Luke Castellan, then he can snap out of the spell. Their true form can paralyze a man just by the sheer sight of them. They use this form to drink blood but may also be used to seduce men as Tammi tries to do so in the battle of the labaryinth. Some empousai have limited metacognition, as they may know that people are watching them in dreams or visions, as shown in a scene where Kelli scares Percy out of his dream. They are able to manipulate the Mist, shape-shift and charmspeak as shown in The House of Hades. Older empousai apparently have the power to escape in a shower of flames.