|Popes Formosus & Stephen VII, by Jean Paul Laurens, 1870|
The synod was called by Pope Stephen VII. He had been advanced in his religious career by a previous pope, Formosus (c.816 - 4 April 896). Pope Formosus was pope for about five years, during which he made a few questionable political moves, like getting involved in the conflict between Charles the Simple and Odo over the French crown and clashing with Holy Roman Emperor Guy III of Spoleto.
After Formosus died, he was succeeded by Boniface VI, who lasted a matter of weeks and was succeeded in May 896 by Stephen VII, who called the Cadaver Synod. The cadaver was Pope Formosus, who was put on trial. How do you put a dead person on trial? You dig him up and put him in a chair at the synod. Since he was a pope, however, you put him in the proper vestments, giving a new twist on "respecting the dead."
It was decided at the synod that he had been unworthy of the papacy due to his actions. All his decrees and decisions were declared null and void. To make the symbolism complete, they ripped off his papal garb, cut off the three fingers of his right hand that had held the consecrated Host, and threw the body into the river.