There's also a possibility that it was a new character introduced in the fanfic. This character could have some special power or knowledge that gave them an edge over All For One. They might be from a different dimension or have a power that is specifically designed to counter All For One's abilities, which led to his injury.
Perhaps it was a group of heroes working together. They combined their powers and strategies to get the better of All For One. Each hero might have contributed their unique abilities, like one hero using their speed to distract him while another used their strength to land a powerful blow, ultimately injuring him.
In the 'All For One Gravely Injured but Escaped Fanfic', it's likely that the plot was set up in a way that put All For One in a vulnerable position. This could be due to a betrayal from within his own ranks or a new, powerful force emerging in the fanfic's universe. His injury was so grave that it seemed like the end for him, but he escaped. Maybe he had a secret passage or a teleportation device that he used at the last moment. He's a smart and resourceful villain, so he'd find a way out even when things look really bad.
It could be a new and powerful villain who has emerged. This villain might have a special ability that catches Naruto off - guard during a fight, leading to his injury.
Maybe it's a new villain in the fanfiction. Some powerful and evil character who sees Leo as a threat to their plans. This villain could use their unique powers or weapons to harm Leo. For example, a villain with the ability to control electricity could zap Leo and cause internal injuries.
It could be a jealous rival. This rival might be envious of Estel's popularity or skills and decides to harm her secretly.
It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.
The Projectionist might be responsible. As the Projectionist is always lurking around in the dark corners of the studio, it could mistake Bendy for an intruder and attack him. Its large and menacing form could easily cause some serious injuries to Bendy.
In some darker fanfictions, it could be a member of the Order who has always suspected Snape of being a double agent. This person might have decided to take matters into their own hands and attack Snape during the meeting, believing they are protecting the Order from a potential threat.
Perhaps it was a result of friendly fire or a mistake by an ally. This would add an interesting layer of drama and conflict, especially if there are trust issues that need to be resolved.
There's also the possibility that it was an accident. Fanfiction often plays with the idea of magical accidents. Maybe a spell misfired during a demonstration or a fight that broke out among the Death Eaters that Snape got caught in the middle of, and as a result, he got injured.
A new and powerful rogue ninja could be responsible. This ninja might have a vendetta against Konoha and see Naruto as a symbol of the village, so they target him and use their strong and devious techniques to injure him badly.