One reason could be a hidden disease or affliction that is unique to the fanfiction world he is in. It could be something that has been passed down in his family magically. Or perhaps he has been in a situation where he was exposed to a harmful magical energy source for too long. This continuous exposure could gradually damage his body and result in coughing up blood. For instance, if he was trapped near a leaking magical artifact that emits harmful rays.
One reason could be overusing his chakra. In Naruto's world, chakra is like life energy. If he overexerts himself and drains too much chakra, it can cause internal damage, leading to coughing up blood. For example, when he was training intensively or during a tough battle where he pushed his limits.
Another possibility is internal injuries from a battle. Fanfiction often has intense battles that are not part of the original story. Harry could have been hit by a powerful spell during a fight that caused internal damage, which then makes him cough up blood.
Well, it could be a result of a dark spell cast on him by a Death Eater. Maybe they found a new and insidious way to harm him without leaving an obvious trace.
Maybe he was poisoned. There are many villains in the Naruto world who use poison. If Naruto was poisoned during a fight or an ambush, it could lead to him coughing blood as his body tries to fight off the toxins. Just like how Sasuke was once poisoned and had to be treated.
It could be due to a dark curse or spell. Maybe he was attacked by a powerful dark wizard using a forbidden spell that affects his internal organs and causes bleeding.
It could be a dark magic curse. Maybe some enemy has cast a spell on Harry that is slowly sapping his health, leading to him coughing blood.
Coughing up blood referred to coughing up blood from certain parts of the respiratory tract in the lungs. Common causes include chronic asthma, dilated bronchi, respiratory diseases, and cardiovascular diseases. The symptoms of hemoptysis may be bright red or pink blood, which may contain mucus. The amount of hemoptysis is usually small, but if there is a large amount or frequent hemoptysis, you should seek medical attention to determine the cause and receive appropriate treatment.
One possible reason could be to create a tragic story. Writers might want to show how the world of the story would be affected by the loss of such a central character like Percy.
He might leave because of a personal conflict. Perhaps he has a falling - out with some of the other campers or with the camp leaders. Like if he disagrees with a new rule or policy that the camp has implemented which he thinks is unjust.
One possible reason could be a dark turn in his character. Maybe he's been corrupted by some external force like a powerful and evil magic. Another reason might be a misunderstanding. For example, he believes the camp has betrayed him in some way, so he lashes out in anger.