lernaean hydra is the offspring of typhon and akkidna
The hydra was trained by the Greek Goddess hera to attack villagers
Once the Hydra has one head cut off, two more grow in it's place. The Hydra has poisonous blood and breath. The Hydra guards the portal to the underworld.
The Hydra lives in a body of swamp water. The Hydra has contaminated the water so no one can drink it or swim in it.
One of Hercules 12 labors was to kill the Hydra. Hercules first attempt to kill the Hydra failed, he did not know two heads would replace the one he had cut off.
Hercules brought his nephew Loreless to help him kill the Hydra. Hercules realized once the head was cut off the other person had to cauterize the neck to prevent the two heads from growing back to replace the one cut off. They cut the immortal head off and buried it in the woods.