I have the idea for this kinda developed in my mind, but I haven't even put together a proper outline yet. This is just to see how the opening sequence turns out before I put more time into it.
Percy frowned. His mouth opened to say something, but he paused before closing it. Several seconds later, he tried again.
"You'll have to run that past me again."
Chiron walked to the other side of the ping pong table, the shuffling of his four hooves being an oddly expressive movement for the centaur.
He turned back to look at the young demigod, grimacing slightly, "You should take a seat."
Percy slumped into a frail wooden chair, rubbing his eyes tiredly. The ancient hero trainer just sighed in response, grabbing something and tossing it across the room. The flying object took the young demigods' attention, and he watched stupidly as a stuffed leopard mounted on the ceiling ate the airborne sausage.
"At the creek last night," Chiron began, regaining his attention, "We learned that your father is Lord Poseidon, God of the Seas and one of the Twelve Olympians, among other things. That is chiefly concerning because he is one of the Big Three, as his child you attract more attention from monsters and other hostile forces."
He took a deep breath before continuing, "Now to explain the rest, I need to tell you a short story, and for that I need your full attention."
He paused to wait for Percy. The newly discovered demigod moved his head upright, training his attention warily on the centaur.
"Thousands of years ago, just after the Gods overthrew Kronos, the original six Olympians realized there were far too few deities allied with them. They needed to make additional Gods to bolster their forces."
"They didn't want to make just anyone a God however, so they devised a competition to find those who were worthy of divinity."
Percy straightened further in his seat, training his exhausted eyes on the ancient trainer of heroes.
"And I'm in it?"
Chiron sighed loudly again, offering back a tired look of his own.
"At first, any mortal who made their way past a few monsters was made a God, but after a few hundred years they already had hundreds upon thousands of deities to deal with so they stopped that. Now, the competition only happens every century, and only certain Demigods can participate."
Percy suddenly stood from the chair, his foot tapping urgently on the floor. "Can I give up my spot to someone else?"
"No." Chiron's answer was certain and clear. Percy dropped limply back into the seat.
"Only certain half-bloods are selected as competitors," he reminded. "One mortal child of every god has to compete."
"A god with multiple children, Lord Hermes for instance, has to pick which one of their offspring will be their champion. If a god has no living mortal children, then they go without a champion for that century. You are Poseidon's only demigod child, so there is no one else to pick; you must be his champion. "
Chiron paused, locking his gaze solemnly on Percy.
"For better or worse, you are a competitor in the Race for Divinity."
Interesting idea or no? I have some ideas to put down, but for now it's late and I want to get some sleep.