The Armor of Light by Melissa Scott and Lisa A Barnett

The heavy summer air of 1595 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.
But this is not quite the England that we know. The Queen's champion Sir Philip Sidney did not die at Zutphen, nor was poet and spy Christopher Marlowe murdered in a Deptford tavern, and both are powerful magicians, if in different traditions. The Queen must send them north to break the prophecy and save the King of Scots - and England's future.

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