The Laidly Worm by Flora Steel

In Bamborough Castle there once lived a King who had two children, a son named Childe Wynde, and a daughter who was called May Margret. Their mother, a fair woman, was dead, and the King mourned her long and faithfully. But, after his son Childe Wynde went to seek his fortune, the King, hunting in the forest, came across a lady of such great beauty that he fell in love with her at once and determined to marry her.