Slaine: The Brutania Chronicles, Book Three

Albion, in a time of legend. Celtic warrior SL AINE united the tribes of the Earth Goddess Danu and became the first High King of Ireland, before returning to being a wanderer. Now, having journeyed to the Isle of Monadh to rescue Sinead from the Drune Lords, he's discovered that they have been experimenting on their own creations...