This Sacred Isle — Steven Smith

Britain AD 593 - Roman rule has long since ended. After years of chaos, Anglo-Saxon kingdoms now dominate most of the island, with the native Britons driven out or enslaved in their own lands. But rumours whisper that Merlin the Sorcerer has returned to unite the Britons and defeat their hated enemy...

Meanwhile, fourteen-year-old Morcar daydreams of battles and heroic deeds. However, when his village is attacked, Morcar is forced to venture into the wild - and there he must confront the danger that threatens to destroy his people.