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:

Bede's writings coincided with the onset of Carolingian Reformation.

Notes There are striking similarities between Emperor Constantine, Charlemagne and Henry VIII.

Constantine had his eldest son executed, as he believed that his son had an affair with his second wife, Fausta. Few month later, Constantine had Fausta also executed.

As Charlemagne was already married to Himiltrude, the Pope objected to his proposed marriage to Desiderata, daughter of the King of Lombardy. Three years later, Charlemagne repudiated Desiderata and married Hildegarde. After Hildegarde death, he married Fastrada.

In the similar vein the marriage of Henry VIII to Anne Boleyn was objected by the Pope as Henry was already married to Catherine of Aragon. Three years later, Henry had Anne executed and the next day he and Jane Seymour were betrothed. After Jane Seymour died, Henry VIII married Anne of Cleves. Here the similarity ends, as Henry Tudor's proclivity for judicial murders of his spouses (Anne Boleyn, Kathryn Howard) is singularly unique.

Constantine legislated the change of the status of coloni (tenant farmers) to that of serfs. He also legislated replacement of crucifixion by hanging (not to lessen the condemned prisoner's suffering but for reasons of religious symbolism), upheld that slave owners can beat their slaves to death or pour molten lead down their throats.

Charlemagne, a half-educated religious fanatic with considerable capacity for brutality legislated the death penalty for offences that included

• Cremation of dead relatives
• Refusal to be baptized
• Non-celebration of Christian festivals
• Being inimical to Christianity
• Having beliefs other than Christianity

Henry VIII the founder of the Church of England, not only executed two of his six wives, but between 1513 and 1547 also 72,000 other persons.