These creatures are the devotees of the goddess Hecate. They can assume the shape of any animal (even human), but whichever shape they take they always wear golden sandals. They devour the living, leaving their bones at crossroads to frighten unwary travellers.
In their human form one of their legs is that of a donkey and the other is made of brass.
One way to get rid of the Empusae is to insult them.