https://fromtheheartofeurope.eu/the-harp-and-the-blade-by-john-myers-myers/

The author is a fascinating figure in his own right; the book is a rollicking Dark Ages story of a bard who turns warrior and defender of the vulnerable, which would qualify as straight historical fiction, were it not for the first chapter in which the Little Folk put the protagonist under a geas which compels him to be helpful. As usual, the names are completely wrong, but the story is a decent fist at imagining historical France in the 6th or 7th century, and, as mentioned already, it is rollicking good fun. ( )

a Druid spell compels a harper to aid fellows

Strictly, this is historical fiction, since there are no overtly supernatural elements, but it has the "feel" of first-class heroic fantasy like Poul Anderson or Fletcher Pratt. To me it is the best of Myers' books except maybe Silverlock. It is set in the reign of the Emperor Otto I of the Holy Roman Empire, but in France, chiefly on the borders of semi-independent Brittany. The hero is a fine Irish warrior-bard, Finnian, who becomes allied with Conan the Breton, possibly ancestor to the dukes of Brittany, currently fighting for those ;lands with a nasty warlord named Chilbert. The action is fast and furious, but Finnian also has time for clever bardic performances. My favorite scene is one where Finnian conveys a messags to a captive in the presence of his enemies using Ogham done with his fingers on his harp. The tale ends with Finnian rejected by the woman he has tried to woo, but he takes it cheerfully and gets drunk. ( )