Perhaps Allen meets the 14th in a dream - like sequence in the early fanfic. There, the 14th could be trying to communicate something important to Allen, like a warning or a piece of crucial information about the future. Allen, on the other hand, would be confused and a bit scared at first but then try to understand what the 14th is all about.
Well, in the fanfic, Allen might have a very intense encounter with the 14th. Maybe there would be a battle of wills between them, as Allen tries to resist the influence of the 14th. It could also involve some revelations about Allen's past or the nature of his Innocence that are related to the 14th.
Perhaps Allen and Neah would have an intense verbal exchange. Allen might question Neah's intentions right away, given Neah's somewhat mysterious nature in the D Gray - Man series.
One possible aspect of an early fanfic where Allen meets Neah could be the exploration of their complex relationship. Maybe it delves into how Allen initially reacts to Neah's presence, whether with suspicion or a strange sense of familiarity.
Sorry, I haven't actually read this specific fanfic, so I can't give a definite summary. But generally, it should be about Allen's encounter with the 14th, which might include some kind of conflict or discovery.
In this brainwashed Allen fanfic, it could also be that the brainwashing is part of a larger scheme. The Millennium Earl might be trying to use Allen as a weapon against the Black Order. Since Allen has the Innocence and his special powers, if he's under the Earl's control, he could cause a great deal of damage. The fanfic could then detail the battles that occur as a result, with the Black Order having to fight Allen while also trying to save him. They might have to find some long - lost knowledge or a special item to reverse the brainwashing process.
The story could explore the origin of the Noah and how Allen's connection to them is deeper than anyone thought. Maybe his family has some hidden ties to the Noah. This discovery could be the catalyst for his transformation. As he becomes a Noah, he might start to uncover secrets about the war between the Exorcists and the Noah that were previously unknown. This could lead to a re - evaluation of the entire conflict in the D Gray - Man universe.
In some fanfics, Allen might be depicted as dead, but it really depends on the specific plot and the author's creativity. Fanfic writers often take liberties with the original story, so there could be various interpretations.
One possibility is that the fanfic focuses on the relationships Allen has in this new state. His friends might be confused or worried about him. For example, Lenalee could be trying to reach out to him, but Allen is pushing her away because he doesn't want to put her in danger. And the Order might be divided on how to handle Allen now that he's different. They could see him as a threat or as a potential weapon against the Millennium Earl.
Perhaps the Earl discovers Allen when he is in a vulnerable state. The fanfic might explore how the Earl's presence affects Allen's development. It could also show Allen's internal struggle between the kindness the Earl might show him and his loyalty to the Black Order. It could be a story full of psychological drama as Allen tries to figure out the Earl's true intentions.
Isolation is also a theme. Allen is often alone in his struggles, being the host of the 14th Noah. Fanfics may depict his feelings of isolation from both the Exorcist side and the Noah side, and how he copes with being different from everyone else.
The most obvious antagonist would be the Millennium Earl. He has the motive and the means to brainwash Allen. He wants to destroy the Black Order and Allen, with his Innocence, is a major threat to the Earl's plans. So, by brainwashing him, he can turn that threat into an advantage.