Maybe he faced an extremely powerful enemy that he couldn't overcome. It could be a new and terrifying monster created by the Titans or some other dark force.
Perhaps he was betrayed. There could be a character who was secretly working against him all along. This character could have led him into a trap or weakened him somehow before the final blow. Another possibility is that he made a mistake in his judgment, like trusting the wrong person or misinterpreting a prophecy, which ultimately led to his death.
The other characters would likely be devastated. Annabeth, his love, would be heartbroken. She might become more withdrawn and determined to avenge him. His friends like Grover would be in deep sorrow, feeling as if they've lost a part of their strength. And the whole demigod community could lose hope, as Percy was seen as a great hero.
It might also be related to some sort of enemy or threat that specifically targets Percy and his relationship with Annabeth. The enemy could be so powerful that Percy realizes the only way to stop it from harming Annabeth is to sacrifice himself, like if the enemy feeds on his life force or his connection to Annabeth, and he chooses to end it by sacrificing his own life.
Some fanfiction writers may do it to stand out. Since the Percy Jackson series is so well - known, creating a version where he is a devil can attract attention. Also, it could be a way to explore darker themes. The original series has a lot of heroic and positive elements, but in fanfiction, making Percy a devil allows for a look at the other side of the coin, like how his powers could be used for evil instead of good.
It could start with Artemis getting into a situation where she's in a vulnerable position. Maybe she's been lured into a place where her powers are diminished. Percy, being a friend and an ally, rushes to her aid. Along the way, he has to fight through hordes of lesser monsters, which takes a toll on him. When he finally reaches Artemis, they are confronted by a really powerful adversary. Percy realizes that the only way to save Artemis is to take the full brunt of the enemy's attack himself. So he does, and that's how he dies.
One great fanfiction could be 'The Hidden Path in the House of Hades'. It explores the relationship between Percy and Annabeth in more depth during their time in the House of Hades. The author adds some really interesting side quests that the characters embark on, which makes it a thrilling read.
Perhaps he has lost too many people close to him. If his friends, like Annabeth or Grover, were to die in a particularly tragic way, he could be so grief - stricken that he no longer wants to live in a world without them. He might think that death is the only way to escape the pain of their loss.
The 'Redemption Arc' is also common. Some fanfictions focus on a character who has made mistakes in the past, like a minor villain, and they get a chance to redeem themselves in the House of Hades setting. It adds an interesting layer of depth to the story.
One reason could be that the writer wants to create a more angsty and dramatic story. Sally's death would be a huge shock to Percy and the readers. Another reason might be to test Percy's character, to see how he would handle such a great loss.
Perhaps Percy has proven his worth through his unwavering loyalty and courage. Loki, who is used to seeing self - serving and deceitful behavior among the gods and mortals, is impressed by Percy's pure - heartedness. His actions of always protecting his friends and standing up for what is right might make Loki feel that he is worthy of respect, leading to Loki bowing to him.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.