Melissa Scott and Lisa A. Barnett’s 1988 The Armor of Light is a standalone historical fantasy novel.
Court astrologer John Dee approaches the aged Queen Elizabeth with disquieting news. If nothing is done, then at some time in the next century a British monarch will be put to the axe. The Virgin Queen is horrified by this monstrous reversal of the natural order.
The key to the affair is James VI, King of Scotland, who even now is the focus of a dire magical plot. In a normal state of affairs, the death of a Scotsman is no great matter (to the English). In the case of James VI, he is the man who will inherit Elizabeth’s crown when she dies. His fate is significant.
Someone will have to travel to Scotland to deal with the matter. That someone is two people: Sir Philip Sidney and Christopher Marlowe.