The Fellowship of the Talisman

Novel, first published: Del Rey/Ballantine 1978

The setting is medieval England in an alternate universe in which the world has remained in the Middle Ages for a thousand years, its progress thwarted by the evil influence of the Harriers.
The hero, Duncan Standish, must deliver a manuscript, historical proof of Jesus's existence, to Oxenford University for authentication. Duncan, reluctant to go but a dutiful son, is soon joined by that heterogeneous crowd that always accompanies Simak's fantasy heroes: a seven-foot giant, a huge killer mastiff, a war-horse, a gray burro, a bashful ghost, a hermit, a goblin, an old witch, a banshee, and the lady Diane mounted on a griffin. Harried by the Harriers, who are aliens from the stars, the company stumbles onto an enchanted castle, where they meet a dotty wizard and an unfrocked demon named Scratch. ... The story ends in the inevitable confrontation between the company and the Harriers, who are really after the manuscript.

Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.115-116