Formosus was one of the most controversial figures of the late-9th-century Roman papacy. He had been consecrated Bishop of Porto (suburbicarian see near Rome) in 864 under Pope Nicholas I, and served as papal legate to the Bulgarian conversion mission of Khan Boris I in the 860s.
Under Pope John VIII (872–882), Formosus was accused of conspiring against the papal authority, deposed, excommunicated, and exiled. He was rehabilitated by Pope Marinus I in 883 and restored to his see of Porto under Adrian III and Stephen V.
On the death of Stephen V in 891, Formosus was elected pope — the first pope to have been bishop of two separate sees before his papal election (Porto, then Rome), a canonical irregularity that would provide the political pretext for his posthumous trial.
His five-year pontificate (891–896) was devoted to the management of the disintegrating Italian-imperial situation: he crowned the Saxon king Arnulf of Carinthia (the successor of Charles the Fat in East Francia) as emperor in 896 — angering the Spoletan party of Lambert of Spoleto (whose father Guy III Formosus had previously crowned).
Formosus died of natural causes on April 4, 896. Nine months later, his successor (after the fourteen-day pontificate of Boniface VI) Pope Stephen VI (896–897) — a partisan of the Spoletan party — exhumed the corpse of Formosus, clothed it in papal vestments, placed it on the papal throne, and subjected it to ecclesiastical trial: the Cadaver Synod (Synodus Horrenda) of January 897.
The corpse was found guilty of perjury, episcopal-translation irregularity (in crossing from Porto to Rome), and usurpation. The three fingers of the right hand (the fingers of papal blessing) were cut off; the vestments were stripped; and the corpse was thrown into the Tiber.
The Cadaver Synod provoked popular revolt in Rome; Stephen VI was deposed and strangled in prison; the Cadaver Synod's decrees were formally annulled by popes (Theodore II, John IX, Sergius III). The episode is among the darkest in Catholic papal history and the opening act of the saeculum obscurum.

