He was a soldier from Byzantium, one of the defenders of Smolensk when it was besieged by the Tatars in 1238. One day the Mother of God appeared to Mercurius and told him that the Tatars were preparing a surprise attack — and, further, that he must take up arms and attack the enemy singlehandedly. Placing all his trust in God, the lone soldier threw himself against the Tatar host crying 'Most Holy Mother of God, help me!' He was quickly surrounded and cut down, and it appeared that his action had been as foolhardy as it had seemed, when a woman at the head of a glorious host, all of them surrounded by light, appeared and threw back the Tatar army. The next morning the people of Smolensk found the ground covered with the bodies of their enemies. They buried Mercurius in the Cathedral, where he has been venerated as a Martyr ever since.