Son of the twins and the Underworld

Don't ask me where this idea came from, it just happened: My name is Percy Jackson. I am seven years old. When I was four, my mother told me about my dad. She said he was a Greek god. She didn't know which one, but he was. A little bit later that day, a big black dog came. I think mommy said it was a hellhound. She took out a sword, but it killed her quickly. I saw two arrows, one gold one silver, go through the dog and it dissolved into dust. A 12 year old girl and 16 year old boy ran to me. (Continued in first chapter)