Ra Ra Pontic King, Lover Of The Caucasus, He Drank It All And Said I Feel Fine!
This is Mithradates VI Eupator. A king who ruled a Greco-Persian state in what is now Northern Turkey called Pontus. It was a descendant of the conquests of Alexander the Great. At a young age he feared being eliminated by his enemies and decided to start taking small doses of things known to be poisonous so as to build up an immunity to them. He developed this into a state sponsored science eventually to see how to do this properly when he was king.
He fought the Roman Republic a number of times, but one time when he lost to the Romans and expected to be captured by them, he feared that if he was, he would be taken to Rome and paraded in a military parade by whatever general took him alive. He didn't want to do that so he tried to drink poison on purpose, but he was in fact successful at preventing assassination by poison because it didn't kill him. The Romans failed to capture him regardless as he had a servant stab him.