Brotherhood of the Gemstone

Nine years after disappearing at sea, during a storm Murchadh the fisherman washed ashore, bequeathing to his son, Cormac this terrible artefact: It along with the quest to the island of Antilia near where Tuathmurdún lies, to destroy the Blood-Gem with the mystical Spear of Light. It's a Scottish tale part way between Lord of the Rings, and Vinland Saga