His friends could be involved. For example, Lucy might stumble upon some old records or meet someone who knew Natsu in the past. Gray could also be part of it, especially if it has something to do with the magic world's secrets that they both are a part of.
It could be a new character introduced specifically for this purpose. A mysterious figure who has been following Natsu's story from afar and has some connection to his past. This character might be a detective - like figure in the magic world, with the sole purpose of uncovering the secrets that Natsu has been hiding. Or perhaps it's an ancient being that has knowledge of events long past and decides to bring Natsu's dark past to light for some grander purpose, like to balance the power in the magic world.
Well, it could be things like a tragic family history. Maybe Natsu's family was involved in some sort of dark magic that got them in trouble. Another possibility is that Natsu had a past encounter with a powerful and evil entity that left a mark on his psyche.
It could make him seem more vulnerable. Usually, he might come across as strong and fearless, but revealing his dark past shows he has his own insecurities and traumas.
It could be someone from Natsu's past. Maybe an old rival who knows about his dark secrets and wants to use them against him.
It could be about Natsu finally opening up about a tragic event from his childhood that shaped his personality. Maybe it involves the loss of a loved one, or an experience that made him distrust a certain group of people.
The 'natsu dark past secret fanfiction' might revolve around Natsu's family. There could be some dark secrets about his family that he has just discovered. Maybe his family was involved in some sort of dark magic or a feud with another powerful group, and Natsu is now starting to uncover the truth about it.
Gray could be one. Since he and Natsu have a long - standing rivalry, Gray might try to bring Natsu back to Fairy Tail. He'd be conflicted between his annoyance with Natsu and his concern for his former guildmate.
The main characters are obviously Natsu and Mirajane. Natsu, who has a dark side in this fanfiction concept, is known for his fiery magic and impulsive nature. Mirajane, on the other hand, is a powerful mage with her demon form capabilities.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
There are a few authors that stand out. One author, let's call them [X], has a knack for writing emotional and in - depth stories about Dark Natsu and Erza. Their fanfictions often focus on the psychological aspects of the characters, which makes for a very interesting read. Then there's [Y], who is known for creating really complex storylines. In their works, Dark Natsu and Erza are often faced with difficult choices that really test their relationship. And of course, [Z] is popular for writing more light - hearted fanfictions with a lot of humor, still maintaining the essence of Dark Natsu and Erza's relationship.
Another idea for this fanfiction could be that they were both students at a very old and secret magic academy in the past. Zeref was an outcast because of her dark magic, but Natsu didn't care. He befriended her, and together they explored the secrets of the academy and the magic within it. However, their friendship was soon tested when the academy's elders discovered Zeref's true nature.