A pope of this name was accused of being a necromancer by one of his enemies, Benno of Meissen. Another pope of this name is 
A pope of this name was accused of being a necromancer by one of his enemies, Benno of Meissen. Another pope of this name is best known for his collaborations with astronomers Aloysius Lilius and Christopher Clavius. Henry IV walked to Canossa in a hair shirt to have an excommunication that was given to him by a pope of this name rescinded during the Investiture Controversy. For his contributions to the development of Catholic liturgy, the first pope of this name is nicknamed "the Great." For 10 points, give this name shared by sixteen popes, including the one who created a namesake calendar that is still in use today.