In this kind of fanfiction, it's often a very tragic and heroic concept. Percy sacrificing himself to save the seven would be a huge shock to the other characters. It might lead to a lot of grief and also a new sense of purpose for the remaining seven. They would have to carry on with their mission while also dealing with the loss of Percy, which could add a lot of depth to their character development.
Percy might become more independent. Without his mom's presence, he has to rely more on himself. He could start to take on more leadership roles within the demigod community. He also may develop a darker side to his personality, as the pain of his mom's death lingers. However, through his experiences and the support of his friends, he gradually learns to balance this new, harsher side with his original kind - hearted nature.
The main emotional impact would be grief. His friends in the seven would be deeply saddened. There would also be guilt for some who might think they could have done more to save him.
One possible plotline could be Percy's journey of grief and how he copes with the loss. He might become more withdrawn at first, then gradually find strength in his friends and his quest. Another could be his search for a way to bring his mom back, perhaps delving into dangerous magic or making deals with powerful beings. Maybe he also has to face the fact that his mom's death was part of a larger plan by the villains and he has to stop them from achieving their ultimate goal.
There could be a fanfiction titled 'Percy's Struggle After Losing His Mom'. In this, it could show Percy having flashbacks of his mom during battles or difficult times, and how those memories both haunt him and give him the will to keep going. It might also include how he tries to honor his mom's memory in different ways.
Percy often becomes more somber. He may lose some of his initial naivete. For instance, he might be more cautious in his actions, thinking twice before jumping into a fight like he used to.
Another theme is growth. After his mom's death, Percy often has to grow up faster and become more independent. Fanfics might show him taking on more responsibilities and maturing in ways he wouldn't have if his mom was still alive. For example, he may become more decisive in battles or in dealing with other challenges. In some fanfics, there's also the theme of legacy. Percy might discover things about his mom that she left behind for him, like a hidden power or a special message. This gives him something to hold on to and also adds to the idea that his mom's influence still lives on even though she's gone.
The 'Seven' could refer to a group of characters within the same fictional universe as Percy. Without more context, it's hard to say exactly who they are, but they are likely a group that has had interactions with Percy and now admires him.
Without more context from the fanfiction, it's hard to say exactly who 'the seven' are. They could be a group of friends, allies, or a special team within the fanfiction's universe. They might be characters with unique powers or abilities that are relevant to the story.