King and Emperor (Hammer and the Cross/Harry Harrison)

Set during an alternate Dark Ages that never was, The Hammer and the Cross is the gripping saga of Shef, the One King, a visionary and warrior whose accomplishments have changed the course of history. First, he led a mighty Viking horde to victory over England. Then he defeated his foes among the Norsemen to become the unquestioned King of the North. Now, in the powerful conclusion to Harry Harrison's acclaimed trilogy, Shef must face the reborn power of the Holy Roman Empire.
Ruled by the German knight Bruno, who wields the Holy Lance with which Jesus Christ was slain, Rome threatens Shef's fearsome Viking navy with a new invention of unparalleled destruction: Greek fire. Unable to defend his fleet against this awesome weapon, Shef travels to the East in search of new wisdom. His quest leads him to the lavish court of the Muslim Caliph and, ultimately, to the secret hiding place of the Holy Grail.
While the ancient Norse gods observe Shef from distant Asgard, visions and portents warn of the coming Ragnarok, the ultimate Twilight of the Gods. Faiths and empires clash in the final battle between The Hammer and The Cross, and not even the gods can predict the outcome.