What is "armilus"

Armilus (also spelled Armilos and Armilius) is an anti-messiah figure in medieval Jewish eschatology, comparable to medieval interpretations of the Christian Antichrist and Islamic Dajjal, who will conquer Jerusalem and persecute the Jews until his final defeat at the hands of God or the true Messiah. His inevitable destruction symbolizes the ultimate victory of good over evil in the Messianic age.