In the Twelve Labors of Hercules, Hercules used a sword to cut off the Lernaean Hydra’s heads.
The Hydra was a many-headed serpent living in the marshes near Lerna. Its central danger was that cutting off one head could cause two more to grow in its place. Hercules therefore needed assistance: his nephew Iolaus cauterized each neck with fire after Hercules severed the head, preventing regeneration.
The final immortal head was buried beneath a heavy rock, and Hercules dipped his arrows in the Hydra’s poisonous blood. Those poisoned arrows later became important in other myths, including Hercules’s fatal conflict with the centaur Nessus and the death of the centaur Chiron in some accounts.
The sword detail is most securely associated with the standard retelling of the labor, although ancient art and later adaptations may depict Hercules using different weapons. The defining feature of the victory is the combination of cutting and burning, not swordplay alone.