The Grey Knight (1986)

From the back cover:

You should know these noble people by their arms. Some you can trust, while others are deadly foes. One is Arthur, king of all England, a good and kindly man. Another is the Grey Knight, a fearsome warrior and the King’s dire foe. He challenges Arthur’s right to rule and accuses him of murdering hundreds of children in northern England.

Sir Gawaine, Arthur’s best knight and champion, will defend the King’s honor in a duel to the death. But Merlin prophecies that Gawaine will die at the hands of the vile Grey Knight, with only one chance to save him.

There are ancient artifacts of extraordinary, magical power. One of the Thirteen Treasures of Britain is the key to Gawaine’s salvation.

Arthur proclaims a solemn quest: comb the length and breadth of England to find these treasures before Gawaine and the Grey Knight meet.

Challenge Death’s Champion!