Hades in Greek mythology is the god of the underworld and the dead. Equivalent to the Roman god Pluto, which means the rich and who was also one of its Greek epithets, his name was often used to designate both the kingdom and the god who rules in the underground of the Earth.
It is considered a god of "second generation" that originated outside of Cronus (Saturn in Roman form) and Rhea, with her five brothers formed the Crônidas: women Hestia, Demeter and Hera, Zeus and Poseidon men.
He is also known to have abducted the goddess Demeter's daughter Persephone, who had been faithful and who never had children. The symbolism of this union puts in communication the two main forces and natural resources: the wealth of the subsoil that provides the minerals, and bring forth the seeds of its heart - life and death.
Hades usually has a secondary role in the mythology, because the fact of being the ruler of the Underworld means that its work is "divided" among other deities, such as Thanatos, god of death. How do you ruthless and invincible death, Hades is the god most hated by mortals, as Homer reported.
Hades had a companion was his dog Cerberus. It looks monstrous, with several heads, the dog was responsible for guarding the entrance from the dead.
Big Three is a collective term that refers to the three main gods of Olympus: Zeus, Poseidon and Hades. The Big Three are equally matched in power for the most part, with Poseidon commanding the seas that cover most of the Earth, and Hades as the god of the Underworld, which represents each person who has died and is now a ghost. Although Poseidon and Hades be a "great power", Zeus commands the sky that covers the entire Earth, but has few inhabitants, while Poseidon and Hades have many more creatures in their domain.
The Big Three are the most powerful gods of Olympus, so your demigod children are the most powerful half-bloods. The half-bloods were causing major problems during the Second World War, with the sons of Zeus and Poseidon on one side and the other children of Hades. The end came when the Oracle predicted that a child demigod of the Big Three would destroy or save Olympus when completed 16 years, then the three gods swore by the river Styx to not have more children with mortals, this pact was called the Covenant of the Big Three .
While all agree that not having more children demigod, the promise was broken by the gods except Hades who already had two children before the oath, Zeus broke his promise and had Thalia, while five years later Poseidon had Percy. Hades with Nico and Bianca does not go against the rules of the oath already born before the oath be made, but were frozen in time, "Lotus Hotel and Casino."
Percy was living with his mother until age 12 and arrived at Camp Half Blood. During the Second World War, Zeus tried to kill Bianca and Nico destroying all the building they were in, with lightning, but the only death was Maria di Angelo, Bianca and Nico's mother and lover of Hades. After that, Nico and Bianca mysteriously disappear until they were found in The Titan's Curse.