Synodus Horrenda, the posthumous trial of Pope Formosus, took place in Rome during January of 897. In his youth, Formosus carried out missionary activities among the Bulgarians. After his return to Rome, Formosus was accused of corrupting the minds of the Bulgarians and attempting to overthrow the Papacy of the John VIII. Formosus fled the Rome, where he returned after the death of John VIII and was elected Pope on October 6, 891.

In 892 Formosus crowned Lambert of Spoleto Emperor. Shortly afterwards Lambert's cousin marched on a Byzantine enclave in Italy. Formosus opposed this war against the Byzantium and invited the Emperor of Carinthia, Arnulf, to invade  Italy. In 895 Arnulf crossed the Alps and entered Rome. Lambert of Spoleto fled the Rome and Formosus crowned Arnulf the Emperor. Arnulf and Formosus both died in 896. After Formosus death, Lambert returned to Rome and instigated criminal proceedings against Formosus.

The body of Formosus was exhumed and seated in a courtroom, where his successor, Pope Stephen VI, conducted the trial. Formosus was found guilty, stripped of its papal vestments, and the fingers of his right hand were cut off. His body was reinterred only to be dug up again, tied to weights, and cast into the Tiber. However, these bizarre proceedings turned the public opinion against Stephen, who was deposed, imprisoned and while in prison, in July of 897, strangled to death.

Pope Formosus trial was described a thousand years later by poet Robert Browning: