All the people in the world are linked to someone by a red thread, which joins the pinkies of both, connecting their souls

And no one seems to notice it, no one more than Perseus. Percy could see everyone's threads, tangled... tense... cut...

His was always there, just hanging from his little finger, as if the other end were far... very far away. But today, he feels a slight tug and it is because the thread is tightened "but what the f-...?"