Hercules was killed by Nessus's blood, which had been smeared on a garment and given to him as a love charm.
Here's a more detailed explanation:
Hercules' wife, Deianeira, became jealous when Hercules fell in love with another woman. She remembered Nessus the centaur's deceptive advice. Before dying, Nessus had told her that his blood would act as a potent love charm. Deianeira, believing this, soaked a robe in Nessus's blood and sent it to Hercules. However, Nessus's blood was actually a powerful poison. When Hercules put on the robe, the poisoned blood burned his skin, causing immense agony. He eventually died from the excruciating pain caused by the poison.
In summary, although given under false pretenses as a love potion, the poison from Nessus's blood ultimately caused Hercules' death.
