|“||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.||”|
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 tried to kill them to prevent them from reaching the age of sixteen, but Hades protected them and sent them to the Lotus Hotel and Casino to hide.
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, which he almost did until Zeus spared her life and made her a tree. Poseidon also had a child a short time after Zeus did.
After the Second Titan War and the passing of the Great Prophecy, the gods felt they had no reason to stay true to the pact as it didn't work in the first place and because strong demigods weren't a bad thing.
Instances of the Pact Being Broken
- Zeus broke the pact first when he saw a TV star, Beryl Grace, who was described as a fluffy blonde starlet. With her he had a child, Thalia Grace.
- 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. (Zeus forgives Percy later). Percy feels that Poseidon thinks that he is a mistake, but will be proud of him.
- Hades had sired Bianca and Nico di Angelo before the pact was made after the end of World War II. However, due to Zeus being scared of any child born of the Big Three, he tried to kill them with his lightning bolt, 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 them before the pact was made.
- 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. However, Hazel was been born before the start of World War II, meaning Pluto still wouldn't have broken the pact.
Reasons for the Pact
- 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 gods would decide the fate of the gods and Mount Olympus, possibly for the worst, with an inevitable choice.
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.
- Athena considered it ironic that Hades, lord of the Underworld, was the only one of the three not to break the oath.
- Zeus was the one who initiated the oath to not have any demigod children and went on great lengths to strongly impose that on his brothers. He killed Hades' lover, Maria di Angelo (in the attempt to kill Hades' children, Nico and Bianca) 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.