Thanks to really dull closing shifts, I found the story of the Cadaver Synod, which may be the most insane thing I've ever read. A pope put his dead predecessor trial, found him guilty, and had him excommunicated. While I can appreciate the spectacle, I don't understand the purpose. What was the pope trying to gain from this trial? What did it accomplish?
Cadaver Synod, definitely an interesting topic. In summary: the synod was called for political reasons to discredit the opposing faction.
Some context: We are dealing with the papacy during the late 800s, probably the worst time for the papacy (much much worse than the renaissance popes like the Borgias). Popes rose and fell in matter of weeks, and were often put on the throne of St. Peter for political reasons by various factions, either Roman noble families of external forces (like the King of the Franks). You can read more about the context at At the time we are duiscussing, there were two factions fighting for the Imperial throne and that looked at the papacy for support: Carolingians (rulers of France) and Sploetians 9rulers of Southern italy). The Holy ROman Emperor had to be corwned by the Pope, so these faction also fought for the papal throne itself.
So this is the story: Pope Formosus (papacy 891-896) is the protagonist of this weird piece of papal history. An earlier pope who was an enemy of Formus, Pope John VII, had crowend Holy Roman Emperor Guy III of Spoleto (part of the Spoletian faction). Pope Formosus (a pro-Carolingian) was uncomfortable with Guy being Emperor and relations deteriorated when Guy forced Formosus to crown his son Lambert of Spoleto co-ruler of the Holy Roman Empire. To fight back against Guy, he invited Arnulf of Carinthia (a Carolingian) to advance to Rome, invade the Italian peninsula, and liberate Italy from the control of Spoleto. It took him two expeditions, but at last Arnulf made it to Rome. Pope Formosus then crowned Arnulf Emperor. yet, both fo them died short after.
Pope Formosus' successor was technically Pope Boniface VI but he reigned only 15 days.b In the meanwhile, in the absence of Arnulf, the Spoletions had invaded Rome.and sponsored the unclear election ofPope Stephen VI, who was a pro-Spoletian.
In order who exhumed Pope Formusus' body and put it on trial. By condemning pope Formosus he condemned the Carolingians and bolstered the power to the Spoletians. Also (this is basically the answer to your question) by excommunicating him he made all his ordinations and decisions void; this meant that Formosus' coronation of Arnulf as Emperor was null, and all the bishops Formosus had ordained, were now void. Basically, pope Stephen could now re-ordained who he wanted (obviously putting people from his faction). Though the trial, pope Formosus and all of his faction were discretited and Pope Stephen could annul everything that they did. Ironically, Stephen VII himself had been ordained by Formosus. To desecrate him even more, the body of pope Formosus was thorwn in the Tiber.
Not surprisingly, the people of Rome did not like the spectlae of the Cadaver Trial. They revolted against Stephen, who was imprisoned and killed.
Hope this helps.
Sources:
[Wikipedia] (https://en.wikipedia.org/wiki/Cadaver_Synod)
[Encyclopedia Britannica] (http://www.britannica.com/topic/Cadaver-Synod)
[Atlas Obscura] (http://www.atlasobscura.com/articles/morbid-monday-cadaver-synod)