“How many other old enemies were in this crowd? Percy began to realize that every battle he’d ever won had only been a temporary victory. No matter how strong or lucky he was, no matter how many monsters he destroyed, Percy would eventually fail. He was only one mortal. He would get too old, too weak, or too slow. He would die. And these monsters … they lasted forever. They just kept coming back. Maybe it would take them months or years to re-form, maybe even centuries. But they would be reborn.
Seeing them assembled in Tartarus, Percy felt as hopeless as the spirits in the River Cocytus. So what if he was a hero? So what if he did something brave? Evil was always here, regenerating, bubbling under the surface. Percy was no more than a minor annoyance to these immortal beings.
They just had to outwait him. Some day, Percy’s sons or daughters might have to face them all over again.”
The House of Hades by Rick Riordan
This is probably the most depressing thought of the series so obviously it’s my favorite part. Children’s book they said.