Hercules used Hydra’s blood as a poisonous coating for his arrows after completing the Hydra labor.
The Lernaean Hydra’s blood was treated in myth as intensely toxic. Hercules dipped his arrows into it, turning ordinary projectiles into weapons capable of causing incurable wounds. This detail links the second labor with later episodes in Hercules’ life and is one of the most important consequences of the Hydra’s defeat.
The poisoned arrows eventually caused the death of the centaur Nessus and, indirectly, the death of Hercules. Nessus used a poisoned garment to deceive Deianira, Hercules’ wife; the garment had been contaminated with Nessus’s blood, which itself carried the Hydra’s poison after Hercules had shot him. The mythology therefore presents the Hydra’s blood as a dangerous object whose effects continued long after the labor.