Well, it depends on the relationships. For example, his mom would be completely shattered. His death could also cause some characters to question their own mortality and the futility of their fight against the gods' problems. Some might turn bitter, blaming the gods for not protecting Percy. It could also lead to a split in the group, with some wanting to continue the fight and others losing their will.
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 She might become more withdrawn and determined to avenge him.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.
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.
Percy's death would shatter the world of those who knew him. His friends like Tyson would be inconsolable. Tyson always saw Percy as a hero and a brother. The Hunters of Artemis might also be affected. They would respect Percy's sacrifice but also feel a sense of responsibility for his death. And the gods, who had often used Percy for their own ends, might start to question their actions. Some might feel remorse, while others might fear the chaos that could follow in the wake of Percy's death, as he was often a stabilising force in the often - tumultuous world of demigods and gods.
Well, it would probably shatter Annabeth. Percy is her closest friend and love. His death would leave a huge void in her life. She might become more withdrawn, consumed by grief. She could also become more determined to carry on Percy's legacy in the fight against the forces they were up against.
His friends would be devastated. Annabeth, in particular, would be heartbroken. Their relationship is very strong, and his death would leave a huge void in her life. She might become more withdrawn and less trusting.
For the other characters, Percy's death could change the entire dynamic. His leadership and strength were often relied upon. Without him, there could be a power vacuum. Some might lose hope, thinking that if Percy couldn't survive, what chance do they have? Others might use his sacrifice as a rallying point, to become stronger and more united in the face of danger. But overall, it would be a very dark and difficult time for all those who knew and cared about Percy.
The other characters would be devastated. Annabeth, for instance, would be heartbroken. She has been through so much with Percy, and his death would leave a huge void in her life. She might become more withdrawn and less trusting. Grover would also be deeply affected. He has been Percy's friend since the beginning, and he would probably blame himself for not being able to protect Percy. The other demigods would lose their hero, and they might feel less confident in their ability to defend Olympus.
On the other hand, she could try to honor Percy's sacrifice. Annabeth might focus on the reasons why Percy died for her and strive to fulfill any tasks or goals that he might have hoped she would achieve. She would carry on his legacy in a positive way, even though she is heartbroken.
In fanfiction where Sally dies, it's a major turning point. Percy's character development could take a different route. Instead of having his mother as his anchor, he has to find strength within himself. This could make him more reckless at times as he no longer has that voice of reason in his life. It might also make the other characters step up more to support Percy, like Annabeth having to be more of a stabilizing force for him. And the story could then focus more on Percy's internal struggle with loss and how he copes with it in the world of demigods.
Percy would be completely devastated. He might go into a state of shock at first, unable to believe that Annabeth is really gone. Then he could become filled with rage, vowing to take revenge on whoever or whatever was responsible for her death.
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.