For the pact of the Big Three, go here.
The Big Three
The Big Three are the three most powerful gods in all of Olympus: Zeus/Jupiter, Lord of the Sky, Poseidon/Neptune, Lord of the Seas, and Hades/Pluto, Lord of the Underworld. In the beginning, after they had defeated their father, Kronos, they drew lots to see who would get where as their domain. Zeus received the sky, Poseidon received the sea, and Hades received the Underworld. Zeus retains control over the sky and the weather, Poseidon can manipulate water, storms, and earthquakes, Hades can summon the undead, control shadows, and use earth-related disasters.
Therefore, the Big Three are equally matched for the most part. The powers of the Big Three are also directly related as Zeus' control over the weather originates from the Earth's seas, which is Poseidon's domain, while Poseidon's control over storms and natural disasters are linked to the weather, which is Zeus' domain. Hades retains his power from people dying while Zeus and Poseidon get their power from people living, fishing, and praying to them. However, people believe that Zeus is the strongest, followed by Poseidon, and then Hades, due to possessing the Master Bolt as his standard weapon and being the King of the Gods.
The Earth is split between the gods and is no one's specific domain, though Zeus is believed to control the earth. However, that is debatable due to Demeter's power over the earth as the Goddess of Agriculture. Besides this, the other gods are also not to be underestimated, because the Big Three only get their raw power from how high they are among the Olympian hierarchy.
A half-blood of the eldest gods
Shall reach sixteen against all odds
And see the world in endless sleep,
The hero's soul, cursed blade shall reap
A single choice shall end his days
Olympus to preserve or raze
Demigod Children of the Big Three
|“||You are children of the three major gods. Who could withstand your combined power?||”|
–Persephone to Thalia, Nico, and Percy in, The Sword of Hades
As said before, all of the Big Three agreed not to have children, but Poseidon and Zeus did. Hades already had three children before they swore the oath, so technically he never broke it. The worst he did was never sending Nico and Bianca di Angelo to Camp Half-Blood and Hazel Levesque to Camp Jupiter. Poseidon has Percy Jackson and a Cyclops named Tyson, however because Tyson is not a demigod, there is no fear of him fulfilling the first Great Prophecy. Zeus had a daughter named Thalia Grace and, as Jupiter, a son named Jason Grace.
Nico and Bianca Di Angelo
Hades already had Nico and Bianca when the Big Three made their agreement not to have any more kids, for fear of the Great Prophecy. Zeus gave Hades a week to send them to Camp Half-Blood, but Hades never did as he knew that Zeus would destroy them the minute they were out of Hades' territory. It was a week after the deadline and Hades was still trying to convince their mother, Maria di Angelo, to let him take them to the Underworld so he could protect them. Maria repeatedly said no, and the last time Hades saw her, she was going upstairs to get her purse. Zeus then struck the building with a lightning bolt, completely destroying it; Hades sensed the attack and had enough time to put a shield around Nico and Bianca but Maria was killed. It was then that he told one of the Furies; Alecto, to take Nico and Bianca to the Lotus Hotel and Casino, where they would be hidden until he sent for them. When they were found, Bianca joined the Hunters of Artemis while Nico went to Camp Half-Blood. However, Bianca died to save Percy and his friends from a defective version of Talos, leaving Nico as the last child of Hades alive. Nico wandered across the world and the Underworld, alone and orphaned, until he discovered Hazel Levesque, his half-sister and resurrected her.
Thalia was the first demigod born after the Oath, but she ended up becoming the tree that guards Camp Half-Blood for almost seven years. She was revived in book two, The Sea of Monsters, when her tree (the source of the magical border around Camp Half-Blood) was poisoned by Luke Castellan. Percy, Annabeth, Clarisse La Rue, Grover, and Tyson retrieved the Golden Fleece from the Sea of Monsters. By mortals, it is known as the Bermuda Triangle.
However the fleece worked too well and Thalia was revived from the tree. She later joined Artemis' Hunters, meaning that she would not age and thus could not be the half-blood in the Great Prophecy.
Percy was the second child of the Big Three to be born after the oath. He is the son of Poseidon and Sally Jackson, and originally did not know anything about his godly heritage. He is probably the most powerful demigod in Camp Half-Blood, along with Thalia and Nico di Angelo. Percy's presence was revealed at the worst possible time, as Zeus' Master Bolt, his symbol of power, and Hades' Helm of Darkness had been stolen. The appearance of a son of Poseidon made Hades and Zeus suspicious, as gods cannot go into each other's territory, but their children can. Poseidon was also the only one of the Big Three that did not have something stolen from him, placing most of the suspicion on him.
Percy found both of the items and returned them, successfully preventing a war. Percy had more adventures and finally saved Olympus from the Titans. Since then, Zeus and Hades have stopped hating him and now merely dislike him. It is also implied that the Big Three might have more children, as stated in The Last Olympian, but they are now allowed to have children as part of the deal they made with Percy.
- Chiron mentioned that three is a holy number, as it is the number of the Big Three, the Furies, the Fates, and the three virgin goddesses.
- During World War II, the Allies which representing the British Commonwealth, the Soviet Union of Socialist Republics, and the United States of America were known as the "Big Three".
- The "Big Three" is also used as a collective name for the three major allies of World War I; David Lloyd George, George Clemenceau, and Woodrow Wilson. It is currently unknown whether they are demigods.
- Interestingly, the Big Three have different eye color each: Zeus has electrically-blue eyes, Poseidon has sea-green eyes and Hades has black eyes. Their hair, in contrast, are similar: and the three of them have black hair