John XII came to the throne in 955, when he was just 18. Shortly thereafter, he turned his residence into a brothel, gambled with church offerings, and was even accused of castrating a cardinal.
German ruler Otto of Saxony once wrote to the young pope, saying, “Everyone [...] accuses you, Holiness, of homicide, perjury, sacrilege, incest with your relatives, including two of your sisters, and with having, like a pagan, invoked Jupiter, Venus, and other demons.”
John XII actually met his demise because of a tryst: A jealous husband discovered his wife in bed with the pope, and the man proceeded to severely beat the him. John XII died three days later.
Source: Royal Scandals