Slaine: The Brutania Chronicles, Book Four

Celtic warrior SLAINE united the tribes of the Earth Goddess and became the first High King of Ireland, but is now a wanderer. Having journeyed to Monadh to rescue Sinead from the Drune Lords, he's discovered that they have been experimenting on their own creations, much to the displeasure of the Cyths' jailer, the Archon Yaldabaoth.