When the hordes of Batu Khan approached Smolensk in 1238, in the service of the Prince of Smolensk was a Roman soldier from a noble Orthodox family, who was Orthodox himself. His name was Mercurius. He was tall, courageous, and strong. It saddened him to see the destruction of so many churches, and he wished to lay down his life for Christ.
One day, during evening prayers in the temple before the wonderworking Smolensk Icon of the Mother of God, the sacristan of the Cathedral heard a voice say, "Go to my servant Mercurius at Podol and quietly say to him: 'Mercurius, the Sovereign Lady is calling you. Go and meet the enemy in your military armor.'
The sacristan went to his house and found Mercurius in the yard, praying in his armor. He had a miraculous premonition, and was waiting for him. The sacristan said, "Mercurius, go at once. The Sovereign Lady is calling you."
They entered the cathedral, and Mercurius made a prostration. Once again, a voice came from the Icon: "My servant, Mercurius, I am sending you to drive the enemy from this city, and to defend this temple. It is for this reason that I have called you from your country. Tonight the enemy has plotted in secret to attack the city, and to destroy it. However, I shall not leave this city; by my prayers, it shall not fall into the hands of its enemies. Go at once to the place called Dolgomoste. An armed host of evil enemies is waiting there. Do not worry; you shall defeat the enemy Voevod. I shall not abandon you. You shall defeat the enemy in battle and obtain a crown of victory from God, and eternal blessedness."
Dolgomoste was 14 versts from Smolensk. There was an enemy force, ready to attack the city in the morning. As he left the cathedral, Mercurius quietly and discreetly passed by the city guards, and with the words, "Most Holy Theotokos help me," he rushed at the enemy. The first soldier was a giant who was proud of his strength, and he was followed by others. The Tatars ran when they saw a radiant Woman, and lightning-fast warriors, who struck them, and so they retreated permanently from the boundaries of Smolensk. Saint Mercurius, however, died in battle, and his head had been cut off.
The next morning, the citizens were surprised to see that the entire field was littered with enemy corpses. They found the body of their protector and buried it in the cathedral church. They hung his armor above his coffin, in accordance with his final wishes, as expressed by the sacristan.
The relics of Saint Mercurius rest in the cathedral church, which is dedicated to the Dormition of the Mother of God.
Saint Mercurius is also commemorated on November 24, the day of his patron Saint, the Great Martyr Mercurius, and on the Sunday before July 15 (Synaxis of the Smolensk Saints).