In 1473, Princess Sophia Paleologos came to Russia with a large retinue to marry Prince Ivan III. One of this retinue was a certain nobleman named Constantine, a relative of the last Byzantine Emperor, Constantine XI Paleologos. Constantine entered into the service of Archbishop Joasaph of Rostov, and in 1489 went with him on his retirement to the Monastery of St Therapont at White Lake. Constantine himself had no plans to become a monk; but on his first night at the monastery St Martinian (January 12) appeared to him, exhorting him to renounce the world immediately. Constantine reported his vision to the Archbishop, who confirmed its authenticity, and Constantine was tonsured a monk with the name Cassian.
The new monk entered into the ascetic struggle with fervor: though he had spent his life as a prince, he gladly undertook the lowliest tasks and the most onerous obedience. At the encouragement of his friend St Nilus of Sora (Nil Sorsky, May 7), he took up a hermit’s life by the River Uchma in the Uglich district. In time various disciples gathered around him, and St Cassian, against his own will, became the head of a skete. His teachings to his disciples always emphasized obedience, care for the poor, and prayer for the departed. He reposed in peace.