The last we saw of Hēraklēs, he was taking a breather and establishing the Olympic Games after capturing the Cretan Bull. Before Hēraklēs returned to vanquishing monsters to clear his soul of miasma, he became involved with two other, major, events: the war with the giants and the liberation of Prometheus. To quote from a post I did on the Giant and Titans:

The rise to power of the Olympians was not an easy one, in fact, under Zeus' leadership the young Gods had many toils and troubles to overcome before They became the dominant force in the universe. Two of those hurdles were the Gigantomachy and the Titanomachy, two events that were first depicted as separate events but became more entwined as the years went on.

The Gigantes were a tribe of one hundred Giants born of Gaia. Some say their father was Tartaros, others that they were born from the blood of the castrated Ouranos. They aren't considered Gods, but fall under the label of 'monster', like the Gorgons and the Hekatonkheires. Hesiod, in his Theogony describes the birth of the Gigantes:

"Then the son from his ambush stretched forth his left hand and in his right took the great long sickle with jagged teeth, and swiftly lopped off his own father's members and cast them away to fall behind him. And not vainly did they fall from his hand; for all the bloody drops that gushed forth Earth received, and as the seasons moved round she bare the strong Erinyes and the great Giants with gleaming armour, holding long spears in their hands and the Nymphs whom they call Meliae all over the boundless earth." [177]

Hēraklēs was alive and mortal for the Gigantomachy, and of course, he fought on the side of the Olympians, earning him great honour with the Theoi for his skill and bravery. Diodorus Siculus writes in his 'Library of History':

"After this, when the Giants about Pallenê chose to begin the war against the immortals, Heracles fought on the side of the gods, and slaying many of the Sons of Earth he received the highest approbation. For Zeus gave the name of “Olympian” only to those gods who had fought by his side, in order that the courageous, by being adorned by so honourable a title, might be distinguished by this designation from the coward; and of those who were born of mortal women he considered only Dionysus and Heracles worthy of this name, not only because they had Zeus for their father, but also because they had avowed the same plan of life as he and conferred great benefits upon the life of men." [4.15.1]

Afterwards, Hēraklēs has one more task to fulfil before he can return to his quest for redemption: free Prometheus. Prometheus, long before was given the task of creating man and his brother Epimetheus was ordered go give good qualities to all creatures of earth. So did Prometheus and Epimetheus. Prometheus shaped man out of clay and Athena breathed life into him. Epimetheus spread swiftness, cunning, fur and wings to the other animals but ran out of gifts when he came to man. Prometheus remedied the situation by allowing men to walk upright and gave them fire.

It soon became apparent that Prometheus loved man more than the Olympians. When Zeus decreed that man must give sacrifice to the Deathless Ones, Prometheus stood ready to aid humanity. He butchered an animal and divided it in to piles; the bones and fat formed one of them, the good meat wrapped in the hide of the animal, the other. Zeus vowed that he would abide by the choice He made now, and picked the tasty looking pile of bones. Zeus was angered but could not take back his vow. What he could take back, was the gift of fire, and this he did.

Mankind suffered greatly without fire and Prometheus travelled either to the sun or Olympus to reclaim fire for his beloved mankind. This, of course, angered Zeus even further and so he devised a plan. First, he imprisoned Prometheus. He ordered Hermes to tie Prometheus to a mountain and had a giant Eagle come every day to eat his liver. As an immortal, Prometheus' liver grew back over night so his torment was endless--well, nearly endless because many, many, many years later, in one version of the myth Hēraklēs took pity on him. Diodorus Siculus again:

"And Zeus, when Prometheus had taken fire and given it to men, put him in chains and set an eagle at his side which devoured his liver. But when Heracles saw him suffering such punishment because of the benefit which he had conferred upon men, he killed the eagle with an arrow, and then persuading Zeus to cease from his anger he rescued him who had been the benefactor of all." [4.15.2]

Next up on the Labours of Hēraklēs series: the horses of Diomedes!