Maria Dolgorukaya was a legendary wife of Ivan IV ("the Terrible") of Russia, possibly 5th or 7th. The marriage (not authorized by the Church) could be celebrated in 1573 or 1580. As the dark legend said, she did not bear the tsar any child and was found out to have a lover (or after the 1st night tsar found out she wasn't a virgin). Ivan had her drowned.
There is no evidence of her existence in the medieval sources. The first mention we can found in the 19th century Russian literature. Nikolay Kostomarov in 1866 wrote that the notice about her was found by Afanasiy Byichkov in some manuscript in Imperial Public Library.
Of all the eight wives of Ivan the Terrible, only Maria Dolgorukaya and Vasilisa Melentyeva (who is also considered as 19th century fake) don't have graves or any mentions in official court documents.