The blood of the centaur Nessus, poisoned by the Hydra's venom, ultimately killed Hercules (Heracles).
Here's a breakdown of how it happened:
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Nessus's Treachery: Nessus attempted to assault Hercules' wife, Deianira, while ferrying her across a river. Hercules shot Nessus with an arrow dipped in the poisonous blood of the Lernaean Hydra, which Hercules had previously slain.
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Nessus's Revenge: As he lay dying, Nessus tricked Deianira, telling her that his blood was a love potion that would ensure Hercules' fidelity. He instructed her to collect some of his blood.
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Deianira's Mistake: Years later, suspecting Hercules of being in love with another woman, Deianira soaked a robe with Nessus's blood (which she believed was a love charm) and sent it to Hercules as a gift.
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Hercules' Agony: When Hercules put on the robe, the Hydra's poison in Nessus's blood began to burn his skin with excruciating pain. The poison was so potent that it could not be washed off.
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Death and Apotheosis: To end his suffering, Hercules built a funeral pyre and had himself burned alive. Some accounts state that he ascended to Mount Olympus and became a god after his death, achieving apotheosis.
In summary, while Hercules was directly killed by the Hydra's poison, it was the blood of Nessus (tainted with that poison) that delivered the killing blow through Deianira's unintentional action.
