Saint Zachariah the Faster of the Caves was an ascetic in the Far Caves in the thirteenth-fourteenth centuries. He fasted so strictly that he ate nothing baked nor boiled, and he consumed only greens once a day at the setting of the sun. Demons trembled at the mere mention of his name.

Often the monk saw angels, with which he desired to live in Heaven. The identification of Saint Zachariah, Faster of Caves, with Zachariah the son of John of Kiev, who had given all his inheritance for the adornment of the Caves temple and became a monk at the monastery, is unfounded.

Before his death, John had transferred his property to his friend Sergius. This was when the igumen was Saint Nikon (March 23). Zachariah was five years old at the time. At age fifteen, i.e., not later than the year 1098, he obtained his inheritance from Sergius, in order to give it to the monastery. However, Saint Zachariah the Faster of the Caves lived approximately 200 years later.

Source:

https://oca.org/saints/lives/2017/03/24/100879-venerable-zachariah-the-ascetic-of-the-kiev-caves