The Royal Changeling

Invading England after Charles II's death, the Duke of Monmouth comes face-to-face with an old friend - Theophilus Oglethorpe. Oglethorpe, in common with only a handful of others, knows Monmouth's background fully - not only that he is Charles's son, but also that he has elven blood, and has allied himself with evil powers in order to take England's crown. In fact, he is being used as a cat's paw by one far more steeped in evil than himself...
King Arthur, who would return to rule Britain and the entire world. Through London, Glastonbury, Sedgemoor and many stranger places unearthly battles rage, and the elves forsake their millennia-long neutrality...