Marcus Varius was a Roman renegade and support of the rebel governor of Spain Sertorius who fought on the Pontic side during the Third Mithridatic War. Mithridates VI of Pontus had attempted to form an alliance with Sertorius before the outbreak of the war, and had gained Sertorius's recognition of his claim to Bithynia and Cappadocia, but probably not to the Roman province of Asia. Sertorius also agreed to send Roman officers to Pontus to help Mithridates train his army.
The leader of these soldiers was a Roman senator, Marcus Varius (given as Varius in Appian but as Marius in Plutarch and Orosius). Once the war began Varius served as one of Mithridates's commanders. After Mithridates's first major victory, over the consul Cotta at Chalcedon, Cotta's colleague Lucullus abandoned his plans to invade Pontus, and instead moved north to relieve Cotta.
While Mithridates moved his main army to besiege Cyzicus, Varius was given command of a detachment that was sent to watch Lucullus. Contact between the two sides was made at Otryae, near Lake Ascania. Battle was about to be joined when according to Plutarch a ' huge, flame-like body', shaped like a wine bottle, and the colour of molten silver, fell between the two armies, which then separated.
Varius next appears in the aftermath of Lucullus's great victory at Cyzicus. Mithridates had attempted to besiege the city, and had then been blockaded by Lucullus. Eventually Mithridates had been forced to abandon the siege, and the Romans had fallen on his retreating army while it was attempting to cross a series of rivers. Varius took command of those survivors who had escaped to Lampsacus, where they were besieged by Lucullus.
Mithridates still had a powerful fleet, which he used to rescue the troops besieged in Lampsacus. He then took most of this fleet back into the Black Sea, but a force of 50 ships and 10,000 picked men, under the joint command of Varius, Alexander of Paphlagonia and Dionysius the eunuch, was sent into the Aegean in an attempt to maintain Pontic command of that sea.
Lucullus responded by raising a fleet from Rome's allies in Asia, with which he hunted down the Pontic fleet. There may have been a clash at Tenedos, but the main battle came on an island close to Lemnos. Lucullus landed part of his army on the island, trapping the Pontic fleet between two opponents. Varius, Alexander and Dionysius were all captured hiding in a cave. Dionysius took poison, Alexander was taken prisoner, but Varius was killed, apparently because Lucullus didn't want his triumph to be marred by the presence of a Roman senator.