Hammer and the Cross Series by Harry Harrison, John Holm

865 A.D. Warring kings rule over the British Isles, but the Church rules over the kings, threatening all who oppose them with damnation. Only the dreaded Vikings of Scandinavia do not fear the priests.Shef, the bastard son of a Norse raider and a captive English lady, is torn by divided loyalties and driven by strange visions that seem to come from Odin himself... more865 A.D. Warring kings rule over the British Isles, but the Church rules over the kings, threatening all who oppose them with damnation. Only the dreaded Vikings of Scandinavia do not fear the priests.Shef, the bastard son of a Norse raider and a captive English lady, is torn by divided loyalties and driven by strange visions that seem to come from Odin himself. A smith and warrior, he alone dares to imagine new weapons and tactics with which to carve out a kingdom--and launch an all-out war between....The Hammer and the Cross. less

A craftsman, visionary, and warrior, Shef has risen from slavery to become king of a mighty Viking nation. But his growing kingdom menaces all of Europe, and he has made many powerful enemies.Chief among his enemies are the Knights of the Lance, a fanatical order of soldiers sworn to bring Shef down, no matter what the cost. To defeat Shef, they will go to extraordinary lengths to find the sacred spear of Christ--and resurrect the Holy Roman Empire... moreA craftsman, visionary, and warrior, Shef has risen from slavery to become king of a mighty Viking nation. But his growing kingdom menaces all of Europe, and he has made many powerful enemies.Chief among his enemies are the Knights of the Lance, a fanatical order of soldiers sworn to bring Shef down, no matter what the cost. To defeat Shef, they will go to extraordinary lengths to find the sacred spear of Christ--and resurrect the Holy Roman Empire.Driven by dreams, Shef battles to change the course of history, but even the gods themselves may be plotting against him.... less

Driven by prophetic dreams, the Viking warrior Shef as become the One King, the undisputed ruler of the North. Now he must face the reborn power of the Holy Roman Empire.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 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.