I read a partially fictionalized account of the “poison king” in which he kept trying to defeat the Roman Empire. With each attempt, he learned a little more about Roman tactics and eventually came to duplicate them himself. He became Rome’s most feared adversary until his rule was usurped and he tried to commit suicide but failed due to his immunity to poison.
Roman general/admieral Pompey defeated M VI, who then attempted suicide by poison, but alas had been (supposedly) acquiring tolerance by taking ever-larger doses.
http://en.wikipedia.org/wiki/Mithridates_VI_of_Pontus#Mithridates.27_antidote