Power being the immemorial fantasy of the powerless, it's unsurprising that modern witches should ask: What if the king were one of ours? What if the king were a witch?
Ever since Margaret Murray, who viewed witchery as a kind of Protest Paganism, first suggested a hundred years ago that the ancient cult of the Sacral/Sacrifical Kingship persisted in the British Isles into early modern times, novelists have asked the question again and again.
Forthwith, a few memorable examples.
The King is a Witch (Evelyn Eaton, 1965)
The year is 1342, the king is Edward III, and yes, he's a witch: our kind of witch, the pagan/Old Religion kind.
Alas, that doesn't mean that he's not a nasty piece of work who spends most of his time looking for divine substitutes to die—in his stead—for the life of the people.
So maybe he's not our kind of witch, after all.
The Devil and King John (Philip Lindsay, 1956)
The king isn't a witch, but his wife is. Bad “my kingdom for a horse” King John, from an Old Craft Revisionist Historical p.o.v.
Well, it's a romp.
King of the Wood (Valerie Anand, 1989)
The king isn't a witch, but his boyfriend is. The life of William II “Rufus”—he of Lammas sacrifice fame—like you've never heard it before.
Let me just mention that his boyfriend, Ralph des Aix, is a horn-wearing King of the Witches himself. He leads the secret (but international) Cult of the Wood, and has a grouping of moles shaped like the constellation Orion on his chest.
Are you in love yet, too?
Watch the North Wind Rise/Seven Days in New Crete (Robert Graves, 1949)
You'll never forget the Midsummer sacrifice of the Antlered King of New Crete, Robert Graves' Goddess-worshiping utopia (but is it?) of the post-apocalyptic future.
Pagan ritual should always be so good.
Dies the Fire (S. M. Stirling, 2004 et seq.)
I'm a sucker for “Witches-rebuild-civilization-after-the-apocalypse” fiction—to my mind, it seems a realistic enough possibility—and Stirling's Emberverse series gets a place of honor in that surprisingly well-populated genre. Hel, at fifteen novels, it gets its own shelf in the section.
In King Artos I of Montival—that's plain old Rudi Mackenzie of Oregon, back home at the covenstead—Stirling aims for a larger-than-life hero in the old Cuchulainn/Achilles/Beowulf mold. His most memorable feat: surviving the unstoppable stampede of a million-strong bison herd by mounting and riding a buffalo bull.
Alas, in the end, this (literally) post-Modern hero simply does not measure up to his counterparts of yore. Heroes engage because, though in some ways larger than life, their flaws nonetheless instill in the rest of us a sense of fellow-feeling. In this way, they inspire us to become better than ourselves. If Cuchulainn, with all his flaws, can be so generous, then maybe I can, too.
Artos, though, has no flaws. Though Stirling, skilled writer that he is, strives mightily to make us like this character, at thirteenth and last, he's simply too perfect. Drop-dead gorgeous, wise, generous, unfailingly fair, good at everything that he does, incapable of losing a fight—he even has a sense of humor—he successfully quests for the magic Sword of the Lady that gives him the ability to read minds and to speak any language fluently, and so (in the end) manages to save the world from an attempted invasion by Cthulhu & Co.
Yes, that Cthulhu.
Lammas Night (Katherine Kurtz, 1983)