Mithridates VI was the King of Pontus (a kingdom near the Black Sea) and one of Rome's toughest enemies. He fought against Rome for 40 years to stop them from taking over the East.
He is very famous for his fear of being poisoned by his enemies. To protect himself, he took tiny amounts of various poisons every day until his body got used to them.
Because of this, he became immune to poison. Even today, the practice of building immunity to poison is called 'Mithridatism' after him.