Date of birth unknown; died about August, 897. Stephen was a Roman, and the son of John, a priest. He had been consecrated Bishop of Anagni, possibly against his will, by Formosus, and became pope about May, 896. Whether induced by evil passion or perhaps, more probably, compelled by the Emperor Lambert and his mother Ageltruda, he caused the body of Formosus to be exhumed, and in January, 897, to be placed before an unwilling synod of the Roman clergy. A deacon was appointed to answer for the deceased pontiff, who was condemned for performing the functions of a bishop when he had been deposed and for passing from the See of Porto to that of Rome. The corpse was then stripped of its sacred vestments, deprived of two fingers of its right hand, clad in the garb of a layman, and ultimately thrown into the Tiber. Fortunately it was not granted to Stephen to have time to do much else besides this atrocious deed. Before he was put to death by strangulation, he forced several of those who had been ordained by Formosus to resign their offices and he granted a few privileges to churches.
http://www.newadvent.org/cathen/14289d.htm
And that's the most positive spin I've seen of him. I like the "Fortunately, he was strangled to death pretty quickly..." line.
An 19th century artist's depiction of the trial:
Let's face it. That's awesome.