Saint Cassian the Greek of Uglich, in the world Constantine, was a descendant of the Greek Mangupa princes. He arrived in Moscow as part of the delegation to Great Prince Ivan III, together with the daughter of the Byzantine emperor, Sophia Paleologa.
Having decided to devote his life to the service of God, the saint declined the offer to remain at the court of the Great Prince, and he resettled near Bishop Joasaph of Rostov. When the bishop withdrew to the Therapon monastery for solitude, Constantine followed him, and he led a strict ascetic life.
He accepted monasticism after a miraculous vision by night of Saint Martinian, urging him to take monastic tonsure. After a certain period of time, Saint Cassian left the monastery going not far from the city of Uglich, near the confluence of the Volga and Uchma Rivers, where he founded a monastery in honor of the Dormition of the Mother of God.
Reports of the monk spread widely, and many people began to come to receive his blessing, to see the wilderness habitation and converse with him. Saint Cassian accepted everyone with love, guiding them on the way to salvation with quiet words.
The monk died in great old age on October 2, 1504. In the Uglich Chronicles many miracles of the saint were recorded, in particular the protection of his monastery from Polish soldiers in the years 1609-1611 by his prayers.
The memory of Saint Cassian of Uglich is celebrated also on May 21, the day he shares with his namesake, the holy Emperor Constantine the Great.