Answer
Pope John IX annulled the acts of the Cadaver Synod, which was a synod convened in 886 by Pope Stephen VI, the successor of Pope Formosus. Stephen VI exhumed Formosus's body, put it on trial, and condemned it to be thrown into the Tiber River. John IX reversed this verdict and had the remains of Formosus interred in the church of St. Peter's.