Outside the ancient Greek city of Argos lay a bottomless swamp called Lerna — the kind of place even brave shepherds went around. Local tradition said it was an entrance to the underworld. Local fact was that something lived in it: the Hydra, a serpentine water-monster with (depending on which vase-painter you trust) anywhere from five to nine heads, breath so poisonous it killed crops, and a habit of dragging cattle and travellers down into the reeds.
Its parents, the poet Hesiod tells us, were Typhon and Echidna — the two beings who between them produced almost every famous Greek monster, from Cerberus to the Chimera. The Hydra was raised, the story goes, by the goddess Hera, specifically so it would one day be a problem for Heracles. Hera was patient about her grudges.
When Heracles arrived on his second labour, the trick of the Hydra became immediately clear. He cut off a head; two grew back in its place. He cut off another; the same. The swamp filled with more serpent than there had been before he started. The labour, as set, was unwinnable by force alone.
Heracles called for help. His nephew Iolaus brought a torch. As Heracles severed each head, Iolaus burned the stump shut with fire before the new heads could sprout. One head — the middle, immortal one — could not be killed at all; Heracles cut it free, buried it under a heavy stone by the roadside, and left it. The Hydra's poisonous blood he kept. It would later coat the arrows that killed (among others) the centaur Nessus, and indirectly Heracles himself.
The Greeks loved this story because it is the cleanest possible parable about the wrong tool for the job. Some problems multiply when you attack them head-on. Some problems need a friend with a torch. Some problems need you to admit that one part of them cannot be solved, only buried — and walked away from.
Wear the Hydra and you wear one of the oldest pieces of Greek wisdom: not every monster is killed with a sword. Some are killed with fire, and a second pair of hands.