Mithradates VI, also known as Mithradates the Great and Eupator Dionysius (134 - 63 BC), was king of Pontus in northern Anatolia (modern-day Turkey). Mithradates was a king of Greek and Persian origin, and claimed descent from Alexander the Great and King Darius I of Persia.
History records Mithradates as one of the more formidable and successful enemies of the Roman republic. He engaged Rome in three separate wars which came to bear his name.
Mithridates VI became king in after his father's assassination in 120 BC. While never confirmed, theories abound that the king was poisoned by his wife, Queen Laodice II. Fearing for his own life, the young king went into hiding and began ingesting small doses of various poisons to build up an immunity. His mother ruled as regent until 115 BC, when the young Mithridates VI took the throne and had the queen imprisoned. She died in captivity later that year. 
After taking power, Mithridates sought to expand his kingdom, entertaining ambitions of making his state the dominant power in the Black Sea and Anatolia. He conquered Colchis, Crimea, Scythia and the Bosporan Kingdom.
The young king then turned his attention to Anatolia, where the Roman power was on the rise. He struck a deal with Bithynian king Nicomedes IV to partition the land, but the latter was simultaneously forging and alliance with Rome. As the relationship between Mithridates and Nicomedes deteriorated, the Bithynian king appealed to Rome for protection. Romans twice interfered into the conflict on behalf of Nicomedes (92 and 95 BC), making a Roman-Pontic war inevitable.
In 88 BC, Rome pressured Nicomedes to invade Pontus and overthrow Mithridates. Mithridates invaded Bithynia and promptly overran the country, leading his troops all the way to the Propontis. After conquering western Anatolia, Mithridates VI reportedly ordered the killing of all Italians living in the area, numbering around 88,000. The massacre of 80,000 Roman men, women and children, which came to be known as the Asiatic vespers, forced Rome into action.
During the First Mithridatic War fought between 88 BC and 84 BC, Lucius Cornelius Sulla forced Mithridates VI out of Greece proper but then had to return to Italy to oppose Roman consul Gaius Marius, in what would erupt into Roman civil war. The Roman-Pontic Treaty of Dardanus officially ended the war in 84 BC. 
The treaty was short-lived as the Second Mithridatic War erupted in 83 BC. According to the Treaty of Dardanus, Mithridates relinquished his land claims in Asia Minor and returned to pre-war borders. Sulla's legate, Lucius Licinius Murena, remained in Asia Minor to ensure the Treaty was respected. However, the ambitious Murena, craving military glory, sought to provoke another war with Mithridates. The war lasted just a year, and Murena was defeated decisively.
In 75 BC, Rome attempted to annex Bithynia, but Mithridates VI attacked with an even larger army, triggering the Third Mithridatic War. Rome sent two prominent generals, Lucullus and then Pompey the Great, against Mithridates VI, who was at last defeated by Pompey in the Third Mithridatic War of 75 BC to 65 BC.
Postwar life and death
After his defeat at the hands of Pompey in 65 BC, Mithridates VI fled to Crimea and unsuccessfully attempted to build a new army to attack Rome. He attempted to commandeer a fleet in Colchis, which was ruled by his son Manchares. Manchares, a Roman client, was unwilling to help his father and Mithridates had him killed. He took his son's throne and continued his attempts to build an army. His order to conscript the Scythians into his ill-fated army led to a rebellion led by his younger son Pharnaces III. In exile and disgrace, Mithridates died in 63 BC. Historical sources differ on whether he committed suicide or was murdered. He was interred in Sinope, the capital of Pontus (modern day Sinop, a city in Northern Turkey).