Hydra or Hydra Lernaean (Greek: Λερναία Ὕδρα) is a myth of ancient Greece, the monster in the form of a snake or a dragon, which has nine heads
Wherever one of head is cut, then the head will grow / form again two new heads. And he took the deadly poison out of his breath every time he stepped.
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Hydra of Lerna was killed by Hercules as the second duty in one of the terms of the "Twelve Olympians" which he had to live for the sake of a penance. In this case, Hercules becomes a candidate of Zeus, and he must compete with Euristheus who is a candidate of Hera.
The Hydra's den is on the swamp of the lerna lake, in Argolid, and as the guardian of the entrance to the world of spirits (the underworld). Hydra is a descendant of Typhon and Echidna (Theogony), both of whom are the disgusting descendants of the earth goddess Gaia.
Narrated, after reaching the swamp near Lake Lerna, where Hydra lived, Hercules covered his mouth and nose with a cloth to protect himself from the toxic fumes that came out of Hydra's breath.
Later, fired a flaming arrow into his nest, the spring of Amymone, which just came out of a very deep cave to terrorize the neighboring villages. Then he confronted him, wielding a sword, he cut his head.
Every head he cuts, grows back into two. Despair pervades the struggle he does. The downside of Hydra, is that only one of its heads is immortal.
Realizing this, Hercules asks for his nephew Iolaus. His nephew came to give an idea (perhaps inspired by Athena), who used a flint to burn Hydra's neck stumps whenever his head was beheaded.
After that, one immortal head of Hydra was placed under a boulder at the sacred ritual spot between Lerna and Elaius, and immersed an arrow in Hydra's poisonous blood, and the second task of Hercules was completed.
In another story it is said that after cutting off one immortal head of Hydra, he dipped his sword in Hydra's blood and used it to burn each head, so that the heads could not grow back.
Knowing that Hercules was assisted by his nephew, Iolaus, in carrying out his second task, Eurystheus declared that the task of Hercules was not finished because he was not alone, and consequently the task of slaying Lernaean Hydra headed 9 was not taken into account in the "Twelve Tasks" set for him.