Three Hearts and Three Lions(1953)

Poul Anderson

The gathering forces of the Dark Powers threatened the world of man. The legions of Faery, aided by trolls, demons and the Wild Hunt itself, were poised to overthrow the realms of light.
And alone against the armies of Chaos stood one man, the knight of Three Hearts and Three Lions. Carlsen, a twentieth-century man snatched out of time to become again the legendary Holger Danske to fight for the world he had helped to build.