In January 897, Pope Stephen VI had the body of Pope Formosus dug up, dressed in papal vestments, and propped up in a chair in St. John's Lateran Church in the Vatican. A defense counsel was appointed for the body, and Stephen then held an ecclesiastical trial charging Formosus with various canon law violations, and the synod of pope and bishops found Formosus guilty.
Formosus' body was stripped of its papal robes, had three fingers cut off to prevent it from giving a papal blessing, and was thrown into the Tiber. Stephen further declared everything Formosus did as pope, including ordinations and consecration of bishops, to be invalid.
The trial was so unpopular, Stephen was put in jail and strangled in his cell in August 897.
In December 897, Pope Theodore II called another synod and invalidated Stephen's synod.
In 898 eithteen-year-old Pope John IX held the first of two synods that supported Theodore and also held Stephen's synod invalid.
But Pope Sergius, who reigned from 904 to 911 invalidated Theodore's and John's synods, reinstated Stephen's synod, and affirmed what Stephen had done.
There has been no official action by the Church since Sergius. His actions have not been challenged by subsequent popes, however, there is no reason to presume anyone wants to do anything except forget the whole thing.
In The Templar Concordat, this is the event which becomes pivotal in the pope's consideration of papal infallibility.