The iron grail / Robert Holdstock.

Fantasy fiction. Seven centuries have passed since Merlin journeyed with Jason and his Argonauts to find the Golden Fleece. Merlin is immortal but when he uses the charm that is knit deep in his bones, his body ages and he has no wish to be old, so rather than squander his magic he prefers to rely on his own intelligence and cunning. Now the mage finds himself in Alba. The Island of Mists beset by enemies both dead and Otherworldly, seeking both the children of the warlord Urtha, who have been kidnapped and taken to Ghostlanda- nd and Jason's younger son, Kinos (Little Dreamer), hidden by his enchantress mother Medea in the Otherworld. So now Merlin must use not just his own cunning and centuries of knowledge but also the magic that permeates his body if he is to save his friends from fates truly worse than death itself.