This Vorontsov was mistress to Grand Princess Catherine as mistress of her husband, Peter III.
Her Wikipedia article includes this, "Catherine II, in her memoirs, pulled no punches when discussing Vorontsova. In a letter from June 1762, she claimed that the Vorontsovs had made plans to shut her up in a cloister and put their relative on the throne. Although Vorontsova wished to follow her lover into exile in Holstein (his homeland), his sudden death put an end to this prospect.
The Empress then arranged her rival's marriage to an army colonel of humble background, and bade them to withdraw to the countryside. There Vorontsova spent the rest of her days in bitterness and ill health, while her brothers Alexander and Semyon made spectacular careers in the diplomatic service."