Before the proceedings the body of Formosus was exhumed and, according to some sources, seated on a throne while his successor, Pope Stephen VI, read the charges against him (of which Formosus was found guilty) and conducted the trial. The Cadaver Synod is remembered as one of the most bizarre episodes in the history of the medieval papacy.
The Cadaver Synod and related events took place during a period of political instability in Italy. This period, which lasted from the middle of the ninth century to the middle of the tenth century, was marked by a rapid succession of pontiffs. In the years surrounding the Cadaver Synod (872-965) there were 24 popes. Often, these brief papal reigns were the result of the political machinations of local Roman factions, about which few sources survive.
Formosus became bishop of Porto in 864 during the pontificate of Pope Nicholas I. He carried out missionary activity among the Bulgarians, and was so successful that they requested him for their bishop. Nicholas refused his permission, because an appointment in Bulgaria would require Formosus to leave his see in Porto, and the fifteenth canon of the Council of Nicaea forbids the transmigration of sees.
In 875, shortly after Charles the Bald's imperial coronation, Formosus fled Rome in fear of then-pope John VIII, for reasons that are not entirely clear. A few months later in 876, at a synod in Santa Maria Rotunda, John VIII issued a series of accusations against Formosus and some of his associates. He asserted that Formosus had corrupted the mind of the Bulgarians "so that, so long as [Formosus] was alive, [they] would not accept any other bishop from the apostolic see.", that he and his fellow conspirators had attempted to usurp the papacy from John, and finally that he had deserted his see in Porto and was conspiring "against the salvation of the republic and of our beloved Charles [the Bald]. Formosus and his associates were excommunicated. In 878, at another council held at Troyes, John may have confirmed the excommunications. He also legislated more generally against those who "plunder" ecclesiastical goods. According to the tenth-century author Auxilius of Naples, Formosus was also present at this council. Auxilius says that he begged the bishops for their forgiveness, and, in return for the lifting of the excommunication, swore an oath to remain a layman for the rest of his life, to never again enter Rome, and to make no attempts to reassume his former see at Porto. This story is doubtful: another description of the synod does not mention Formosus’ presence and says instead that John confirmed his excommunication.
After the death of John VIII in December of 882, Formosus' troubles ended. He reassumed his bishopric at Porto, where he remained until elected pope on 6 October 891. Yet this earlier quarrel with John VIII formed the basis of the accusations made at the Cadaver Synod. According to the tenth-century historian Liutprand of Cremona, Stephen VI asked Formosus' corpse why he "usurped the universal Roman see in such a spirit of ambition" after the death of John VIII, echoing John VIII's own assertion that Formosus had tried to seize the papal throne while he was alive. Two further accusations were also made against Formosus at the Cadaver Synod: that he had committed perjury, and that he had attempted to exercise the office of bishop as a layman. These are related to the oath Formosus is said to have sworn before the council at Troyes in 878.
The Cadaver Synod is generally presumed to have been politically motivated. Formosus crowned Lambert of Spoleto co-ruler of the Holy Roman Empire in 892; Lambert's father, Guy III of Spoleto, had earlier been crowned by John VIII. In 893 Formosus, apparently nervous about Guy's aggression, invited the Carolingian Arnulf of Carinthia to invade Italy and receive the imperial crown. Arnulf's invasion failed, and Guy III died shortly afterwards. Yet Formosus renewed his invitation to Arnulf in 895, and early the next year Arnulf crossed the Alps and entered Rome, where Formosus crowned him Holy Roman Emperor. Afterwards the Frankish army departed, and Arnulf and Formosus died within months of each other in 896. Formosus was succeeded by Pope Boniface VI, who himself died two weeks later. Lambert and his mother, the empress Angiltrude, entered Rome around the time that Stephen VI became pope, and the Cadaver Synod was conducted directly afterwards, at the beginning of 897. The dominant interpretation of these events until the early twentieth century was straightforward: Formosus had always been a pro-Carolingian, and his crowning of Lambert in 892 was coerced. After the death of Arnulf and the collapse of Carolingian authority in Rome, Lambert entered the city and forced Stephen to convene the Cadaver Synod, both to re-assert his claim to the imperial crown, and perhaps also to exact posthumous revenge upon Formosus.
This view is now considered obsolete, following the arguments put forth by Joseph Duhr in 1932. Duhr pointed out that Lambert was in attendance at the Ravenna Council of 898, convened under John IX. It was at this proceeding that the decrees of the Cadaver Synod were revoked. According to the written acta of the council, Lambert actively approved of the nullification. If Lambert and Angiltrude had been the architects of Formosus’ degradation, Duhr asked, “how...was John IX able to submit to the canons which condemned the odious synod for approbation of the emperor [i.e., Lambert] and his bishops? How could John IX have dared to broach the matter...before the guilty parties, without even making the least allusion to the emperor’s participation?” This position has been accepted by another scholar to write about the matter, Girolamo Arnaldi. Arnaldi argued that Formosus did not pursue an exclusively pro-Carolingian policy, and that he even had friendly relations with Lambert as late as 895. Their relations only soured when Lambert's cousin, Guy IV, marched on Benevento and expelled the Byzantines there. Formosus panicked at the aggression and sent emissaries into Bavaria seeking Arnulf's help. Arnaldi argues that it was Guido IV, who had entered Rome along with Lambert and his mother Angiltrude in January of 897, who provided the impetus for the synod.
Formosus was accused of transmigrating sees in violation of canon law, of perjury, and of serving as a bishop while actually a layman. Liutprand and other sources say that Stephen had the body stripped of its papal vestments, cut off the three fingers of his right hand used for benedictions, and declared all of his acts and ordinations (including his ordination of Stephen VI as bishop of Anagni) invalid. The body was finally interred in a graveyard for foreigners, only to be dug up once again, tied to weights, and cast into the Tiber River.
According to Liutprand’s version of the story, Stephen VI said: "When you were bishop of Porto, why did you usurp the universal Roman see in such a spirit of ambition?” and had a laudatory epitaph inscribed on the tomb of Stephen VI.