Wikipedia reports:

"Probably around January 897, Stephen (VI) VII ordered that the corpse of his predecessor Formosus be removed from its tomb and brought to the papal court for judgement. With the corpse propped up on a throne, a deacon was appointed to answer for the deceased pontiff.

Formosus was accused of transmigrating sees in violation of canon law, of perjury, and of serving as a bishop while actually a layman. Eventually, the corpse was found guilty.

Liutprand and other sources say that Stephen had the corpse 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) VII 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) VII said: "When you were bishop of Porto, why did you usurp the universal Roman See in such a spirit of ambition?"[16]

The macabre spectacle turned public opinion in Rome against Stephen. Rumors circulated that Formosus' body, after washing up on the banks of the Tiber, had begun to perform miracles. A public uprising led to Stephen being deposed and imprisoned. While in prison, in July or August of 897, he was strangled.

In December 897, Pope Theodore II (897) convened a synod that annulled the Cadaver Synod, rehabilitated Formosus, and ordered that his body, which had been recovered from the Tiber, be reburied in Saint Peter's Basilica in pontifical vestments. In 898, John IX (898—900) also nullified the Cadaver Synod, convening two synods (one in Rome, one in Ravenna) which confirmed the findings of Theodore II's synod, ordered the acta of the Cadaver Synod destroyed, and prohibited any future trial of a dead person.

However, Pope Sergius III (904—911), who as bishop had taken part in the Cadaver Synod as a co-judge, overturned the rulings of Theodore II and John IX, reaffirming Formosus' conviction,[17] and had a laudatory epitaph inscribed on the tomb of Stephen (VI) VII."