There are 3 books in the The Warlord Chronicles series. The newest book is Excalibur which came out in 1997.

Uther, the High King of Britain, has died, leaving the infant Mordred as his only heir. His uncle, the loyal and gifted warlord Arthur, now rules as caretaker for a country which has fallen into chaos - threats emerge from within the British kingdoms while vicious Saxon armies stand ready…
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Enemy of God

Enemy of God
The continuing story of Arthur, the second in a trilogy which began with THE WINTER KING. The novels bring Arthur and his world to vivid life. A man battling for his vision of the future in a brutal age, dragged down by suspicions and magics of the past, surrounded by…