There was a second prophecy that only Poseidon knew where his firstborn son, Theseus, would become so evil. Poseidon tried to stop the prophecy from coming true by keeping Theseus at the palace with him but was instructed by his aides to kill the boy. They said it wasn't wise, nor safe, to keep the boy alive and risk the lives of many. Poseidon did not listen. He kept the boy alive and hidden away for many, many years.
Poseidon ended up having another son, Percy so that Sally would not be left alone and heartbroken with the idea that her first son was dead.
Theseus was raised on strict ruling. Poseidon made him live his life thinking that there was no other world/city/people than under the ocean, inside the palace. He only heard 'stories' growing up about a hero who saved the world repeatedly (Percy). Theseus was a bad kid, rebellious pre-teen, and a dangerous teen/adult.
One day, Theseus overheard a private conversation Poseidon had talking about how Percy denied the offer of becoming a God.This confused Theseus since he thought this Percy Jackson guy died a long, long, long time ago. He then snuck into Poseidon's private bedroom and saw photos of said Percy and Sally and put everything together. He confronted Poseidon about his new findings, thus having Poseidon confess. Theseus grabbed his belongings and escaped to the surface, leaving no trace behind him. Poseidon hadn't seen him since and was unsuccessful in tracking him down.
Several years passed, and Theseus wanted more. Theseus knows that he has leverage over Poseidon since he's a secret child of the big three. This leads him to get drunk on the power that his existence gives him over the other gods. He wanted to be compensated for all the time he spent locked under the ocean. He wants to be the most powerful demigod, if not God. He wanted immortality, whether it meant turning himself into a god or by other means; either way, he wasn't going to let anybody get in his way. Theseus spent the last few years studying up on Percy, his quests, his friends, where he lives, everything about him. Theseus finds out about Camp Half-Blood and that that is where Percy is staying. Instead of confronting him head-on, Theseus devised a plan to hurt Percy from the inside. He joins camp half-blood.
