The Flaming Chalice Book 3: The Siege of Harlech by Peter Williams
In the aftermath of an apocalyptic disaster, Harlech Castle is home to a growing community of survivors bereft of the comforts and protection of modern technology. The inhabitants possess an ancient chalice which brings them hope, but each night troubles their minds with terrifying dreams of a merciless army growing in the north. The visions turn to reality when the refugees find themselves under siege from a horde of ruthless savages led by a madman claiming to be the new king of England and who will stop at nothing to claim their magical possession. Their only hope is that the faith and compassion which the cup represents can bring them the strength to overcome their terrifying adversary.