In the Twelve Labors of Hercules, Iolaus helped Hercules defeat the Lernaean Hydra.
Iolaus was Hercules’ nephew and a trusted companion. When Hercules cut off one of the Hydra’s heads, two more could grow in its place according to the myth. Iolaus solved this problem by cauterizing each neck with burning brands after Hercules severed the head.
The collaboration made the monster vulnerable. Hercules handled the physical combat, while Iolaus prevented the regenerative heads from returning. Hercules also buried the Hydra’s immortal head beneath a heavy rock and used its poisonous blood on his arrows.
The assistance had an important consequence. Eurystheus later ruled that the Hydra labor did not count as completed by Hercules alone because Iolaus had helped him. This decision contributed to the traditional total of twelve labors after two earlier tasks were rejected or disputed.