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Who was Guibert of Ravenna and why did he become Antipope Clement III?

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Gregory VII excommunicating Henry IV — St. Gregory VII Pope Gregory VII, after his expulsion from Rome, laying a ban of excommunication on the clergy “together with the raging king” (Henry IV of Germany), drawing from the 12th-century chronicle of Otto of Freising; in the library of the University of Jena, Germany.

Guibert of Ravenna (born c. 1025) was a nobleman who served as the imperial chancellor for Italy and later as the archbishop of Ravenna. He became a central figure in the conflict between the papacy and the Holy Roman Empire, eventually serving as the antipope Clement III from 1080 to 1100.1

His rise to the position of antipope was driven by his role as the Italian leader of the imperialist faction that opposed the Gregorian reform movement. After clashing with Pope St. Gregory VII, who subsequently excommunicated him, Guibert was selected by Henry IV to challenge the pope's authority.12

In June 1080, a synod convoked by Henry at Brixen declared Gregory VII deposed and elected Guibert as pope. Following Henry's military conquest of Rome in 1084, Guibert was enthroned as Clement III and proceeded to crown Henry as emperor.12