Read: Apollodorus Labor 2
In those days there was a beast living in the swamps of Lerna that ravaged the countryside devouring cattle. It was known as the Hydra. For his second labor, Eurystheus ordered Hercules to rid the world of this predatory monster.
Taking his nephew, Iolaus (a surviving son of Hercules' brother Iphicles), as his charioteer, Hercules set out to destroy the beast. Of course, Hercules couldn't simply shoot an arrow at the beast or pummel him to death with his club. There had to be something special about the beast that made normal mortals unable to control it.
The Lernaean Hydra monster had 9 heads; 1 of these was immortal. If ever one of the other, mortal heads was cut, from the stump would immediately spring forth 2 new heads. Wrestling with the beast proved difficult because, while trying to attack one head, another would bite Hercules' leg with its fangs. Ignoring the nipping at his heels and calling upon Iolaus for help, Hercules told Iolaus to burn the neck the instant Hercules took a head off. Searing prevented the stump from regenerating. When all 8 mortal necks were headless and cauterized, Hercules sliced off the immortal head and buried it underground for safety, with a stone on top to hold it down. (An aside: Typhon, the Nemean Lion's father, was a perilous underground force, too. Hercules was often pitted against chthonic dangers.)
Having dispatched with the head, Hercules dipped his arrows in the gall of the beast. By dipping them Hercules made his weapons lethal.
Having accomplished his second labor, Hercules returned to Tiryns (but only to the outskirts) to report to Eurystheus. There he learned that Eurystheus denied the labor because Hercules had not accomplished it on his own, but only with the help of Iolaus.
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