While slaying the Nemean Lion was the first labor it was not originally going to be. Even though Eurystheus assigned the tasks the truth is that Hera was telling Eurystheus what to do. When Hera found out he was going to perform tasks as self imposed penance she thought to try and get him killed. She approached Hecate goddess of witchcraft and monsters.
With Hera was Iris goddess of the rainbow and Hera's messenger. Unlike Hera Iris was on good terms with Heracles. Hecate offered two choices the Nemean Lion who was enormous had an impenetrable hide as well as razor sharp claws and fangs or the Lernaean Hydra which was a nine-headed Dragonlike creature that could breathe fire, was venomous on its fangs and blood. The monster also grew two heads for each one cut off. Hera immediately chose the Hydra.
Thinking quickly Iris used reverse psychology telling Hera how kind it was. Hera wished to know what Iris meant so she explained that the Hydra was so dangerous it would kill Heracles immediately whereas the Lion would maul and bite Heracles before finally killing him. Hera's desire to see Heracles die painfully caused her to change her mind. So, Hera told Eurystheus to kill the Nemean Lion and return with its pelt.
The Nemean Lion, son of the monster Orthus and Khimaira dwelt in a two mouthed cave two miles outside of Nemea., north of Argus. The lion was said to be bigger than Heracles, could not be harmed by any weapon. He used to emerge from two openings in his cave and destroy anything and anyone who stood in his way, and terrorized the nearby valleys. As a first task, Eurystheus told Heracles to bring him the skin of the Nemean Lion.
Heracles climbed the mountain, passed the bushes and at twilight he found the lion. Heracles took his bow and shot the lion. After that he tried hitting it with his sword but this did nothing. Heracles once again tried firing arrows and one of them landed between the Lion's feet and he noticed Heracles, and jumped toward him. Heracles tried hitting the beast with his Olive Wood club which caused a ringing in its ears and the lion retreated back to its lair.
Heracles used his enemy's blindness, and blocked the rear entrance of the cave, and stormed through the front, there he found the lion. The hero jumped on the beast's back, and since he could not harm it with weapons, he suffocated it to death. Using the dead Lion's own claws, he opened its body and made a mantle from the skin. Since then Heracles wore it into battle until his mortal death.
Returning with the Lion Pelt to the capital city, his uncle was so afraid of Heracles' mighty power he ordered him to never enter the capital and to leave proof of his success outside the walls of the city and gave his nephew tasks using a messenger.