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About sixty years ago, after World War II, the Big Three agreed they wouldn't sire any more heroes. Their children were just too powerful. They were affecting the course of human events too much, causing too much carnage. World War II, you know, that was basically a fight between the sons of Zeus and Poseidon on one side, and the sons of Hades on the other. The winning side, Zeus and Poseidon, made Hades swear an oath with them: no more affairs with mortal women. They all swore on the River Styx.

Grover Underwood explaining the pact to Percy, in The Lightning Thief.

The sons of Kronos making the oath.

The Pact of the Big Three was an oath made by the Big Three gods Zeus, Poseidon, and Hades, to no longer have demigod children. However, Zeus and Poseidon failed to achieve this and bore Jason Grace and Thalia Grace as well as Percy Jackson respectively. The oath didn't have too much negative effect on them since they were gods and immortal beings.

History

Sometime around the end of World War II, the Oracle of Delphi delivered a prophecy stating that the next child of the Big Three to turn sixteen could be responsible for saving or destroying Olympus. The Big Three had also been watching the war below, as it was fought between their demigod children and they agreed that they were influencing mortal events too much. Because of the potential danger of the Great Prophecy, and the fact that World War II was fought between their children, they vowed to have no more children with mortals.

Despite agreeing to the deal, Hades already had two children under the age of sixteen, Bianca and Nico di Angelo. Zeus, in order to make sure they didn't turn on the gods, ordered Hades to bring them to Camp Half-Blood so they could be trained, but Hades knew it was really so they could be watched. When the deadline passed, Zeus blew up their house to kill Nico and Bianca. Hades managed to save his two children but wasn't able to save their mother, Maria di Angelo, in time. Hades later sent them with Alecto, one of his Furies with their memories washed, to the Lotus Hotel and Casino to hide.

Bianca di Angelo, daughter of Hades

As the decades passed, the Big Three stayed true to their oath, but eventually Zeus fell in love with a movie star and they had Thalia Grace. Upon his discovery of Thalia, Hades—who was still bitter over Maria's death and shocked at his younger brother's hypocrisy—was outraged and sent the most dangerous monsters of the Underworld out to kill her. As Thalia was dying, Zeus made her into a pine tree protecting the borders of Camp Half-Blood. Poseidon also had a child a short time after Zeus did. Zeus, in his Jupiter incarnation, also had another child, Jason, with Thalia's mother. As Pluto, Hades had another preexisting child, Hazel Levesque, who sacrificed herself in the 1940s to stop Gaea from raising Alcyoneus.

As the Second Titan War started, the existence of Poseidon's son Percy Jackson became common knowledge after Zeus' Master Bolt was stolen by Luke Castellan, the son of Hermes. Poseidon was forced to claim his son in order to protect him and Percy succeeded in retrieving the Master Bolt. A year later, Luke and Kronos caused the resurrection of Thalia by forcing the camp to send a quest for the Golden Fleece by poisoning her tree, the Golden Fleece being powerful enough to heal and revive Thalia when it was used to save her tree. Hades subsequently had Alecto release his children from the Lotus Hotel, but Bianca was killed fighting Talos after joining the Hunters of Artemis with Thalia subsequently choosing to join the Hunters as well and to never reach the age of sixteen.

Following the Battle of Manhattan, the gods felt that they had no reason to stay true to the pact as it hadn't worked in the first place and that strong demigods weren't a necessarily bad thing because they had actually saved Olympus. As part of his wish for saving Olympus, Percy Jackson, the demigod of the prophecy, relieved them of their obligation to it. While no other children of the Big Three have been known to have been born since the pact's dissolution, Hazel was resurrected by her half-brother Nico while the Doors of Death were open.

Instances of the Pact Being Broken

Thalia Grace, first child of the broken oath

  • The second was Poseidon who fathered Percy with Sally Jackson. He tells Zeus that it was his wrongdoing so when Percy returns Zeus his Master Bolt in The Lightning Thief, Zeus allows Percy to live for the time being but is still mistrustful of him, and Zeus forgives Percy later. Percy still feels that Poseidon thinks that he is a mistake but will be proud of him.

    Percy Jackson, son of Poseidon

  • Hades had sired Bianca and Nico di Angelo before the pact was made after the end of World War II, therefore meaning he did not break the oath. However, due to Zeus being afraid of any child born of the Big Three, he tried to kill them with his lightning bolt, but failed, only managing to kill their mother, Maria di Angelo, due to Hades's interference. In order to protect his children, he had their memories washed in the River Lethe and sent them into the Lotus Hotel and Casino where they would age slowly; giving Hades a chance to control the prophecy. As Nico and Bianca were born before World War II—and thus before the oath was made—Hades was the only one of his brothers who did not to break the pact. In The Lost Hero, Annabeth claims that they have all broken the pact but Hades had Nico and Bianca before the pact was made.

    Nico di Angelo, son of Hades

  • While still unclear if the Roman gods had a similar pact to their Greek forms, Zeus and Hades (in the form of Jupiter and Pluto respectively) each had a child: Jason Grace and Hazel Levesque with Jason being born to the same mother as Thalia. However, Hazel had been born before the start of World War II, meaning Pluto still wouldn't have broken the pact.

Reasons for the Pact

There are three main reasons why the Big Three Pact was made by Zeus, Poseidon, and Hades after World War II:

  • They wanted no more disasters or wars caused by their demigod offspring, as their children were too powerful.
  • They didn't want any demigod offspring that would be powerful enough to overpower them.
  • But most importantly, the Oracle prophesied that a child of the Big Three would decide the fate of the gods and Mount Olympus, possibly for the worst, with an inevitable choice.

Roman Gods

Jason Grace, son of Jupiter

While still unclear if the pact was made at this point in the series, the Roman gods may not have had to make the same pact as their Greek counterparts. None of the Roman campers mention any kind of limitation on children for the gods during The Son of Neptune. Also, when Percy arrives at camp, none of the other campers are shocked to see a son of Neptune like the Camp Half-Blood campers were in The Lightning Thief. At the same time, none of the other campers think it is strange that there are two children of Pluto (Nico pretends to be a son of Pluto while at Camp Jupiter) so the Pact of the Big Three may not be in effect for their Roman forms.

Trivia

  • Athena considered it ironic that Hades, lord of the Underworld, was the only one of the three not to break the oath because he is usually the one to do the wrong things.
  • Zeus was the one who initiated the oath to not have any demigod children and went to great lengths to strongly impose that on his brothers. He killed Hades's lover, Maria di Angelo (in the attempt to kill Hades's children, Nico and Bianca di Angelo), and resented Poseidon for siring Percy. Yet Zeus was the first to have broken the oath by siring Thalia Grace. Hades, however, did not actually break the oath, as Nico and Bianca were born before World War II. Poseidon broke the oath as well, though Zeus wasn't even aware of Percy's presence in New York.
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