Between 73 and 63 BCE, King Mithridates VI of Pontus was at war with the Roman Empire. He was eventually defeated and his kingdom became an allied state of Rome.
He retreated and left his son in charge. Mithridates was getting ready to raise another army and fight the Romans again, but his son wouldn't help him. Mithridates had his son killed and claimed the throne again.
But, Mithridates' warmongering was not popular, and his younger son led a popular rebellion against him. Devastated by this betrayal, he decided to kill himself with poison.
Only… it didn't work. He had been so paranoid all his life about being assassinated, that he had taken small doses of all known poisons. He had become immune! He ended up ordering his bodyguard to kill him.