Sinner and Saint - English Dawn, #4

Britannia is overrun by migrants who speak a strange language and worship heathen gods, and Bishop Brice is given the job of trying to persuade the Romans to send a legion to drive them out. But the Romans have their own problems and are reluctant to spare any of their dwindling forces. However, a bishop called Germanus, eager to suppress the Pelagian heresy, manages to put together a small force to help the Britons.
His religious zeal leads to a great victory at Maes Garmon - but can it protect him from the wiles of the Angle angel, Rowena? This is the fourth part of the English Dawn pentalogy, which tells the story of the conquest and settlement of Britain by the Anglo-Saxons in the 5th century, set against the broad sweep of events in the Dark Ages: the decline of the Roman Empire, the rise of Attila the Hun, the raids by the Picts and the Scots, and the usurpation of the kingship of Britannia by the tyrant, Vortigern.
These events are a prequel to the age of Arthur, and the final chapter of the pentalogy sees Merlin setting up the sword in the stone for the king who is to come.