Bernard Cornwell - Enemy of God

Enemy of God is the second novel of the Warlord series, and immediately follows the events described in The Winter King. In that book the King of Dumnonia and High King of Britain, Uther, dies and is succeeded by his lamed baby grandson, Mordred. Arthur, a bastard son of Uther's, is appointed one of Mordred's guardians and in time becomes the most important of those guardians. Arthur is determined to fulfil the oath he swore to Uther that Mordred, when he comes of age, will occupy Dumnonia's throne.
Arthur is also determined to bring peace to the warring British kingdoms. The major conflict is between Dumnonia and Powys, but when Arthur is invited to marry Ceinwyn, a Princess of Powys, it seems that war can be avoided. Instead Arthur elopes with the penniless Princess Guinevere and that insult to Ceinwyn brings on years of war that are ended only when Arthur defeats King Gorfyddyd of Powys at the Battle of Lugg Vale. Powys's throne then passes to Cuneglas, Ceinwyn's brother, who, like Arthur, wants peace between the Britons so that they can concentrate their spears against the common enemy, the Saxons (the Sais).
The Winter King, like the present book, was narrated by Derfel (pronounced Dervel), a Saxon slave boy who grew up in Merlin's household and became one of Arthur's warriors. Arthur sent Derfel to Armorica (today's Brittany) where he fought in the doomed campaign to preserve the British kingdom of Benoic against Frankish invaders. Among Benoic's refugees who return to Britain is Lancelot, King of Benoic, whom Arthur now wants to marry to Ceinwyn and place on the throne of Siluria. Derfel has fallen in love with Ceinwyn.
Derfel's other love is Nimue, his childhood friend who has become Merlin's helpmate and lover. Merlin is a Druid and the leader of the faction in Britain that wants to restore the island to its old Gods, to which end he is pursuing a Cauldron, one of the Thirteen Treasures of Britain, a quest which for Merlin and Nimue far outranks any battle against other kingdoms or invaders. Opposing Merlin are the Christians of Britain, one of whose leaders is Bishop Sansum who lost much of his power when he defied Guinevere. Sansum is now in disgrace and serving as Abbot of the Monastery of the Holy Thorn at Ynys Wydryn (Glastonbury).
The Winter King ended with Arthur winning the great battle at Lugg Vale. Mordred's throne is safe, the southern British kingdoms are allied and Arthur, though not a king himself, is their undisputed leader.