“…this see of St. Peter always remains unblemished by any error”—Vatican I, Session 4 (July, 1870), cap. IV.6.

…A man of exceptional intelligence, exemplary life, and strict asceticism (the only fault alleged against him was ambition), Formosus had bitter and relentless foes who included Lambert, once more ruler of Rome , and his own successor Stephen VI, and who did not scruple to subject him to the most macabre humiliation. Nine months (Jan 897) after his death they had his decaying corpse exhumed and, propped up on a throne in full pontifical vestments, solemnly arraigned at a mock trial presided over by Stephen VI himself; a deacon stood by answering the charges on his behalf. He was found guilty of perjury, of having coveted the papal throne, and of haven violated the canons forbidding the translation of bishops. His acts and ordinations were pronounced null and void, and his body (the three fingers of his right hand which he had used to swear and bless having been hacked off) was first placed in a common grave and then flung into the Tiber. A hermit subsequently retrieved and reinterred it.

…While Stephen’s participation in this gruesome affair [The Cadaver Synod] can be explained by his near-hysterical hatred, it is evident that he personally profited by the nullification of Formosus’s acts since the resulting cancellation of his own cancellation of his own consecration as Bishop of Anagni swept away any objections that might be raised, under the canon law of the time, to his elevation to the papacy.

In the following months Stephen was active in requiring clergy ordained by Formosus to produce letters renouncing their orders as invalid. His appalling conduct, however, did not long remain unpunished. A few months later there was a popular reaction, and the outraged supporters of Formosus encouraged by reports of miracles world by his humiliated corpse, perhaps interpreting the sudden collapse of the Lateran basilica as a divine judgment, rose in rebellion, deposed Stephen, stripped him of his papal insignia, and threw him into gaol [jail], where he was shortly afterward strangled.

…With co-operation from the emperor, who controlled Rome and most of Italy, [John IX] at once continued Theodore’s policy of restoring the order in the confused situation arising out of Stephen VI’s trial of the dead Formosus and the ensuing violent clashes between Formosans and anti-Formosans. He convened a synod at Rome, attended also by bishops from north Italy, which annulled the cadaver synod’s sentence on Formosus and burned his acts; those who had taken part in it were pardoned after pleading that they had done so under duress; only Sergius and five close associates are deposed and placed under a ban. The trial of dead persons was prohibited in future. Formosus’s ordinations were recognized as valid, as was his anointing of Lambert as emperor; but his anointing of Arnulf, King of the East franks, as emperor was rejected as ‘barbaric’ as as having been extracted from him by force….

—J. N. D. Kelly, s.v., “Formosus,” “Stephen VI,” and “John IX,” Oxford Dictionary of Popes