HEIR TO A THRONE-OR TO A GRAVEWhen it became certain that Edward VI was dying, the duke of Northumberland, who had been ruling England in his name, made a plan that would let him hold onto his power. He dared not let Mary come to the throne because she was fiercely Catholic and he had espoused the Protestant cause. And he did not want Elizabeth to rule because he knew her imperious nature would never defer to him. But there was more than one puppet master at work: The evil elf lord Vidal Dhu had