Pope Stephen IX was poisoned by the Romans, who were unhappy with his reforms. He was poisoned by a group of Roman nobles who had been opposed to his reforms, and he died a few days later. The pope's death caused great turmoil in the church, and it was not until the next year that a new pope was elected.
Unveiling Pope Stephen IX: A Quiz on the Illustrious Leader of the Catholic Church in the 11th Century