One of the original 12 apostles, Peter was personally chosen by Jesus to be the “rock” upon which he would build his church, according to the Gospel of Matthew. The Roman Emperor Nero, who scapegoated Christians for calamities including the Great Fire of Rome in A.D. 64, ordered their persecution and Peter’s arrest. While the first pope was on the run, he reportedly received a vision to return to Rome and accept his fate to become a martyr. According to accounts written centuries later, Peter asked to be crucified upside down because it would result in a longer, more painful death and not directly emulate Jesus. Saint Peter’s Basilica was built upon what some historians believe is the tomb of the first pope.

In A.D. 258 the Roman Emperor Valerian ordered the executions of Christian bishops, priests and deacons, and among the first killed under the decree was Sixtus II. As the pope was sitting in his episcopal chair addressing his congregation inside a Roman cemetery on August 6, 258, imperial troops stormed the liturgical service and beheaded the pontiff along with four deacons. The martyred pope was later elevated to sainthood, and his feast day is observed every year on the August 6 anniversary of his death.

While popes in the early centuries of the Church were murdered because of their political beliefs, palace intrigue was more likely to cause the untimely ends of pontiffs by the Middle Ages. John VIII was the first pope to be assassinated during a particularly turbulent century that would see multiple claimants to the papacy and a succession of violent papal deaths. John VIII was killed by one of his own clerics—perhaps even by a relative—who poisoned his drink and, growing impatient at the slow pace of the pope’s demise, bashed him in the head with a hammer to finish the bloody job.

The Holy See became so bedeviled by politics that Stephen VI (sometimes referred to as Stephen VII) put one of his predecessors on trial—in spite of the inconvenient fact that the defendant had been dead for nine months. Stephen VI ordered the exhumation of Pope Formosus, dressed the corpse in pontifical robes and sat it on the throne to stand trial for perjury and other crimes. Not surprisingly, the defense was lacking, and the “Cadaver Synod” found Formosus guilty. The three fingers that Formosus had used to deliver blessings were hacked off, and his body was dragged through the streets of Rome before being thrown into the Tiber River. The macabre trial outraged the public, in particular the supporters of Formosus, which delivered its revenge by imprisoning Stephen VI, stripping him of his papal insignia and strangling him to death in August 897.

After ruling for 14 years, John X met his demise after running afoul of Marozia, Rome’s most powerful noblewoman and the alleged mistress of one of his predecessors, Pope Sergius III. After John X asserted his independence and entered into a pact with a new king of Italy, Marozia and other nobles revolted. Their forces killed the pope’s beloved brother right in front of his eyes in the Lateran Palace, and six months later they deposed and imprisoned the pontiff himself in Castel Sant’Angelo, an ancient fortress built by Emperor Hadrian along the Tiber River. John X died in early 929 after being suffocated by a pillow. With John X removed, Marozia installed a pair of papal puppets before the illegitimate son she allegedly had with Sergius III was elected as Pope John XI.

John XII didn’t concern himself much about living a life of chastity and resisting carnal temptations. “John XII scandalized even the Roman society of his day with his addiction to pleasure and debauchery,” writes John W. O’Malley in his book “A History of the Popes: From Peter to the Present.” The pope’s opponents even accused him of turning the papal palace into a brothel. His end may not have been grisly, but John XII died on May 14, 964, doing what he loved. The 27-year-old pope suffered a fatal stroke, allegedly while in bed with a married woman.

There were multiple claimants to the papacy following the death of Pope John XIII in 972. With the backing of Holy Roman Emperor Otto I, Benedict VI was elected pope the following year. When Benedict’s protector died a few months into his papacy, however, an anti-German faction in Rome revolted. They imprisoned Benedict VI inside Castel Sant’Angelo and installed antipope Boniface VII on the Chair of St. Peter. At Boniface’s direction, a priest strangled Benedict VI to death in June 974. Boniface VII himself died under suspicious circumstances in 985 and was so unpopular at the time that his corpse was stripped of its vestments, dragged naked through the streets of Rome and deposited in front of the papal palace where crowds trampled his body and stabbed it with spears.

A decade after the death of Benedict VI, John XIV assumed the papal throne with the backing of Holy Roman Emperor Otto II. While in Rome to direct the papal succession, the emperor contracted malaria and died in the arms of the new pope. Left without his protector, John XIV was arrested, beaten and imprisoned in Castel Sant’Angelo under the orders of Antipope Boniface VII. After languishing for months inside the fortress, John XIV reportedly died of starvation on August 20, 984.

Lucius II met a rocky ending less than a year after his election as pope. Faced with an uprising of the Roman Senate, which declared a constitutional republic free from papal rule, Lucius II raised a small army and attacked the resisters on Capitoline Hill. During the battle, Lucius II was struck by a heavy stone, and the Bishop of Rome died soon after on February 15, 1145.

There may not have been foul play involved in the demise of the first and only Portuguese pope, but the death of John XXI was both painful and bizarre. The only medical doctor to ever lead the Catholic Church, John XII had a small study built for himself at the papal palace in Viterbo, Italy—about 50 miles north of Rome—shortly after his papal election. The decision proved fatal when the study’s hastily constructed ceiling fell on him and caused serious injuries that resulted in his death on May 20, 1277.