His reign as pope was troubled, marked by interventions in power struggles over the Patriarchate of Constantinople, the Kingdom of West Francia, and the Holy Roman Empire.
Because he sided with Arnulf of Carinthia against Lambert of Spoleto, Formosus's remains were exhumed and put on trial in the Cadaver Synod.
[4] On April 19, John VIII called a synod which ordered Formosus and other papal officials to return to Rome.
When Formosus did not comply, he was removed from the ranks of the clergy and excommunicated on the grounds that he had deserted his diocese without papal permission, and had aspired to the position of archbishop of Bulgaria.
Additional charges included the accusations that he had opposed the emperor; "conspired with certain iniquitous men and women for the destruction of the Papal See"; and had despoiled the cloisters in Rome.
[1] As early as 872 he was a candidate for the papacy; Johann Peter Kirsch suggests that the pope may have viewed the cardinal as a potential rival.
[4] Shortly after his election, Formosus was asked to intervene in the Patriarchate of Constantinople, where Photius I had been ejected and Stephen I, the son of Emperor Basil I, had taken the office.
The corpse was disinterred, clad in papal vestments, and seated on a throne to face all the charges from John VIII.
The papal vestments were torn from his body, the three fingers from his right hand he had used in blessings were cut off, and the corpse was thrown into the Tiber, later to be retrieved by a monk.
Sergius demanded the re-ordination of the bishops consecrated by Formosus, who in turn had conferred orders on many other clerics, causing great confusion.