The Armor of Light

From the back cover of the Baen first edition: "A Battle of Wizardry From A History Not Quite Our Own... God save the King of Scotland - The heavy summer air of 1593 is full of portents for Elizabeth, England's Queen and James VI, King of Scotland. A coven of witches secretly controlled by the Wizard Earl of Bothwell has summoned a storm to sink the ship that bears James' bride to Scotland. Though the ship made port, the success of their summoning has emboldened them; the coven is now launching wizardly attacks on the King himself - and James is terrified. From Satan's Wizard Earl - The King has his protectors - not least the Queen of England herself - but not even Elizabeth's secret service and her champion, the redoubtable Sir Philip Sidney, can be on watch every instant. Eventually a trap is sprung, and King James is at the mercy of the satanic Bothwell's deadly spells..."