The Knights of Hades were a group of demon knights from Mephisto’s Realm. The spell-wielding exorcist Elspeth Cromwell unleashed them on Earth to attack a family of suspected witches, actually Mr. Fantastic, Invisible Woman, and their son Franklin. Whatever control she had over them was lost when the blood of an innocent, Franklin, was shed in the ensuing conflict, at which point they simply went on a rampage. Presumably, they were pulled back into the Netherworld, either when Mephisto pulled the Richardses and Cromwell into his realm, or when the demon-lord was defeated by Franklin Richards. They have not been seen since.