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.
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 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.
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.
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.
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.
One popular 'natsu is dark fanfiction' might be those that explore Natsu's inner turmoil in a more sinister way. For example, a story could depict him being corrupted by a dark magic that changes his personality. He might become more aggressive towards his friends, not out of true malice but because of the influence of the dark power. This can create a lot of conflict and drama within the Fairy Tail universe.
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.