Saint Isaac was the first person in northern lands to live as a fool for Christ. His name in the world was Chern. Before becoming a monk, he was a rich merchant in the city of Toropets in the Pskov lands. Having distributed all his substance to the poor, he went to Kiev and received the monastic tonsure from Saint Anthony (July 10).

He led a very strict life of reclusion, eating only a single prosphora and a little water at the end of the day. After seven years as a hermit, he was subjected to a fierce temptation by the devil. Having mistaken the Evil One for Christ, he worshipped him, after which he fell down terribly crippled. Saints Anthony and Theodosius took care of him and nursed him. Only after three years did he begin to walk and to speak. He did not wish to attend church, but he was brought there by force.

Upon his return to health he took upon himself the exploit of holy foolishness, enduring beatings, nakedness and cold. Before his death he went into seclusion, where again he was subjected to an onslaught of demons, from which he was delivered by the Sign of the Cross and by prayer.

After his healing he spent about twenty years in asceticism. He died in the year 1090. His relics rest in the Caves of Saint Anthony, and part of them were transferred to Toropets by the igumen of the Kudin monastery in the year 1711. The Life of the Blessed Isaac was recorded by Saint Nestor in the Chronicles (under the year 1074). The account in the Kiev Caves Paterikon differs somewhat from that of Saint Nestor. In the Great Reading Menaion under April 27 is the “Account of Saint Isaac and his Deception by the Devil.”

Source:

https://oca.org/saints/lives/2017/02/14/100520-venerable-isaac-the-recluse-of-the-kiev-near-caves