When Emperor Decius once waged war against the barbarians, there was in his army the commander of an Armenian regiment called the Martenesians. This commander was named Mercurius. In battle, an angel of the Lord appeared to Mercurius, placed a sword in his hand, and assured him of victory over his enemies. Indeed, Mercurius displayed wonderful courage, mowing down the enemy like grass. Following this glorious victory Emperor Decius made him chief commander of his army, but envious men reported Mercurius to the emperor for being a Christian, a fact which he did not hide but openly acknowledged before the emperor. Mercurius was tortured harshly and at length; he was cut into strips with knives and burned with fire. An angel of God appeared to him in prison and healed him. Finally, the emperor proclaimed that General Mercurius be beheaded in Cappadocia. When they beheaded him, his body became as white as snow and emitted a most wonderful incense-like fragrance. His miracle-working relics healed many of the sick. This most wonderful soldier of Christ suffered for the Faith sometime between the years 251 and 259 and took up his habitation in the Kingdom of his King and God.
For the vision that St. Basil the Great had of St. Mercurius the Great Martyr, and the deposition of Julian the Apostate, see: http://full-of-grace-and-truth.blogspot.com/2009/12/vision-of-st-basil-great-and-julian.html.
Apolytikion in the Fourth Tone (amateur translations below)