In some cases, it could be Naruto's own inner demons. He has been through so much in his life, and all the traumas and pressures he has faced could manifest as fear within him. Maybe in the fanfic, his past experiences resurface and become the source of his fear.
It could be an old enemy who has come back with new and more terrifying powers. For instance, Orochimaru might return with some kind of jutsu that can directly target Naruto's mind and create unfathomable fears in him.
A Naruto fanfic can be for a good cause if it promotes positive values like friendship, just like how Naruto values his friends. For example, a fanfic could show characters from different villages coming together to help each other in a time of crisis.
One possible reason could be that they had a past traumatic experience related to explosions. Maybe they witnessed an explosion that caused harm to their loved ones or themselves. Another reason might be that explosions in the 'Naruto' world are often associated with powerful and dangerous jutsus or attacks, and they are simply afraid of the destruction and chaos that explosions can bring.
One reason could be a past bad experience. Maybe he had a really painful treatment as a kid that left a deep mark. For example, if he had a serious injury once and the doctor was rough while treating him, it would make him fear doctors.
One possible reason could be that his sister has some special or unknown powers that Naruto doesn't understand. For example, if she has a very unpredictable and potentially dangerous kekkei genkai. Another reason might be that she has a very strong and intimidating personality, which makes Naruto feel small and scared in her presence.
This fanfic concept could also lead to new alliances. Characters who oppose the use of fear toxin might team up to stop Naruto, while others who believe in his cause might support him. It would create a lot of political and social unrest in the Naruto universe.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.
Madara Uchiha could be the cause. Even if he's dead, his influence could still be around. Maybe he left behind some sort of genjutsu or curse that affects Naruto's mind and drives him crazy. After all, Madara was always planning something big and Naruto would be a great pawn if he's out of control.
It could be Orochimaru. He's always been interested in Naruto's power. He might use some new jutsu or experiment on Naruto to make him go berserk so he can study his power in that state.
His body language could show it. For example, he might tremble slightly, his hands could be clenched, or he could have a worried look on his face. This would be a simple and direct way to show his fear to the readers.
A 'naruto earth grudge fear fanfic' might be a work where the author explores some dark and less - explored aspects of the Naruto universe. It could be that the 'earth grudge' is a long - held resentment or negative energy associated with the earth in the Naruto world. This grudge then spreads fear among the ninja. It could also involve a character who discovers this 'earth grudge' and has to deal with the resulting chaos and fear, perhaps even trying to find a way to stop it from spreading further.