Hades, like his brothers and sisters, was swallowed at birth by his father, the Titan ruler Cronus, to prevent the fulfillment of a prophecy that one of his offspring would grow up to replace him on the throne. Years later his younger brother Zeus (who had been hidden away by their mother to prevent him from also being swallowed) made Cronus vomit up his siblings, and then led them in a battle to overthrow the Titans.
Knowing they would need armor, weapons, and troops to win the war, the three brothers, Zeus, Poseidon and Hades, traveled to the Underworld to release the Cyclopes from their captivity.
The Cyclopes were a tribe of one-eyed giants who were fine metal smiths. Cronus had imprisoned them in Tartarus, a region in the Underworld that functioned both as a prison and as a place of exile and punishment, rather like our modern vision of hell. The Cyclopes, grateful for their release, crafted gifts for the brothers - thunderbolts for Zeus, a trident (three pronged spear) for Poseidon, and a magic helmet that rendered the wearer invisible for Hades.
The war was long and bloody, and Hades fought ferociously on the battlefront and proved himself a valiant warrior. The younger generation finally won when Hades, wearing his helmet of invisibility crept up on Cronus, Poseidon pinned him down with his trident, and Zeus rendered him unconscious by striking him with a thunderbolt.
Once the battle with the Titans was over, the brothers drew lots to determine which regions each of them would rule. Poseidon won the oceans, Zeus the sky (which made him the supreme ruler over all the gods and goddesses as well), and Hades drew the Underworld.
This seemed to suit Hades just fine. The Greek god Hades, ruler of the Underworld, spent little time with his Olympian siblings, preferring instead to withdraw to his own space and to mind his own business, so to speak.
And surely his divine responsibility was large -- the management of the Underworld, an underground kingdom wherein lived the spirits of those who had died, those who slept and dreamed, and others who, for whatever reason, had been banished from the earth. With his dark, somewhat morbid nature, the Greek god Hades was undoubtedly well-suited to his career. Nonetheless, at times he was bound to be lonely.