Hercules was the Roman name for the greatest hero of Greek mythology -- Heracles.
Like most authentic heroes, Heracles had a god as one of his parents, being
the son of the supreme deity Zeus and amortal woman. Zeus's queen Hera was
jealous of Heracles, and when he was still an infant she sent two snakes to
kill him in his crib. Heracles was found prattling delighted baby talk, a
strangled serpent in each hand.
When he had come of age and already proved himself an unerring marksman with a bow and arrow, a champion wrestler and the possessor of superhuman strength, Heracles was driven mad by Hera. In a frenzy, he killed his own children. To atone for this crime, he was sentenced to perform a series of tasks, or "Labors", for his cousin Eurystheus, the king of Tiryns and Mycenae. By rights, Hercules should have been king himself, but Hera had tricked her husband Zeus into crowning Eurystheus instead.
As his first Labor, Heracles was challenged to kill the Nemean lion. This was no easy feat, for the beast's parentage was supernatural and it was more of a monster than an ordinary lion.
Hercules had twelve labors he had to save people or kill beasts his last one was, As his final Labor, Heracles was instructed to bring the hellhound Cerberus up from Hades, the kingdom of the dead. The first barrier to the soul's journey beyond the grave was the most famous river of the Underworld, the Styx. Here the newly dead congregated as insubstantial shades, mere wraiths of their former selves, awaiting passage in the ferryboat of Charon the Boatman. Charon wouldn't take anyone across unless they met two conditions. Firstly, they had to pay a bribe in the form of a coin under the corpse's tongue. And secondly, they had to be dead. Heracles met neither condition, a circumstance which aggravated Charon's natural grouchiness.
But Heracles simply glowered so fiercely that Charon meekly conveyed him
across the Styx. The greater challenge was Cerberus, who had razor teeth,
three (or maybe fifty) heads, a venomous snake for a tail and another swarm
of snakes growing out of his back. These lashed at Heracles while Cerberus
lunged for a purchase on his throat. Fortunately, the hero was wearing his
trusty lion's skin, which was impenetrable by anything short of a thunderbolt
from Zeus. Heracles eventually choked Cerberus into submission and dragged
him to Tiryns, where he received due credit for this final Labor.