Constantine II, Antipope

Antipope Constantine II (d. 769?) was an Italian man who claimed the title of Pope in Rome in 767. His papacy was not recognized by the most powerful Lombard authorities, and in spring 768 they deposed and imprisoned him. He was blinded in August after a rebellion attempted to restore him to power. In April 769, the Lateran Council had Constantine's tongue cut out for perjury. He was returned to prison, but the manner of his death is unknown.

Constantine II in "Islands in the Sea"
In AD 769, at the request of the heathen Bulgar Khan Telerikh, Pope Constantine II sent Fathers Niketas and Theodore, and Brother Paul to describe the merits of Christianity. Telerikh was considering converting along with his people.

Literary comment
The story never states that Pope Constantine II is the same person as the historical Antipope Constantine II. Constantine was a respected name and could have been adopted as a reign name by any new Pope. However, given Harry Turtledove's use of the "reversal of fortune" trope in several works, the wiki administrators feel safe in identifying him as such.