The story is narrated by George Cranleigh, a younger son of Lord Harold Cranleigh, a destitute landowner in Surrey, who has been ruined, according to Blackmore, by the "farce of Free-trade."
In the opening chapter George, riding home from market, surprises a maiden of surpassing beauty upon her knees in a ruined chapel. She proves to be Dariel, the daughter of Sur Imar, a prince of the Lesghians, a wild tribe of the Caucasus. A blood feud has arisen between Imar and his sister, and so he has, with his daughter, his foster-brother Stepan, and a body of retainers, come to England and settled peaceably in a deserted house in Surrey.
Imar resolves to returns to his native land to educate his tribesmen in the lessons of civilisation. George, who has fallen in love with Dariel, follows her to the East. But Imar's twin-sister Marva, Queen of the Ossets, who is appropriately called by the natives "the Bride of the Devil," plans to kill Prince Imar and wed his daughter Dariel to her son. After weeks of travelling and days full of desperate adventure, George, with the help of miners and Lesghians, rescues Dariel and her father and kills the wicked Princess and her fiendish son.

Question: Who is Marva queen of?
Answer: the Ossets
Alternative answer: Queens of the Ossets