A Greek word meaning avenger, it was used as a title of Zeus in His role as the avenger of evil deeds, especially killings within a family. Later, the name was given to any spiritual entity that would punish wrongful human behavior. Still later, it became an insult similar to the concept of a scoundrel. In Christian terms, Alastor was a generic name for certain evil spirits, and specifically for a possessing spirit. Some have referred to it as the “evil genius of a house.”