Thessalonica

Thessalonica is a standalone historical fantasy novel by Harry Turtledove and published by Baen in 1997. It is set in a universe closely paralleling our own but one where henotheistic gods impact human affairs. It tells the story of the siege of the Christian city of Thessalonica by Slavs and Avars. Its protagonist, George, obtains the aid of several survivors of the endangered Greek pantheon, including centaurs and satyrs, to assist the Christian God and saints against the Slavic gods.