Vasilisa Melentyeva was the legendary sixth wife of Ivan the Terrible. The marriage (not authorized by the Church) could be celebrated in 1575 or she was simply concubine. Modern scholars now considered her as the 19th century fake.
According to the dark legend, before her marriage to Ivan, Vasilisa is recorded to have been a widow of a prince serving in the Livonian War. Though Ivan considered her beautiful and sweet natured, he discovered her a few months after their marriage having an affair with a prince named Devletev. Ivan forced Vasilisa to watch her lover be impaled, and as further punishment, confined her to life in a cloister.
Of all the eight wives of Ivan the Terrible, only Maria Dolgorukaya (who is also considered as 19th century fake) and Vasilisa Melentyeva don't have graves or any mentions in official court documents.
There is no evidence of her existence in the medieval sources except two miser mentions: the 1st, cited by Karamzin, simply listed her name "as concubine" with other Ivan's spouses.