If you've visited FFN recently, you've probably run across at least a couple of Ash betrayal fanfics. They're very popular right now, and all follow the same basic formula: Ash is betrayed by his friends and traveling companions, goes off by himself to do some solo training, then busts back on the tournament scene as a much more powerful trainer, usually with some new love interests in tow, all this to show that those losers had only been holding him back all along.
Ash betrayal fanfic was particularly interesting to me because I had no idea where it came from. It felt like I just logged on to FFN one day to find these stories all over the place, having completely missed whatever set off the trend. It's also unusual in having a very regimented story structure: there's the betrayal, the retreat into training, the tournament that catalyzes Ash's return. There are other elements that are common in Ash betrayal stories, including aura and/or psychic powers, legendary pokémon, and a wide variety of shippings, but the plots all follow a very similar arc. This suggests that there was a single story, what you might call "fanfic zero," that was the prototypical Ash betrayal 'fic and which ended up spawning a ton of imitators. That in mind, I decided to try and track it down to see if I could find out how the genre started and why it remains popular several years later.
Betrayal 'fic in general has been a staple of the fandom since the very beginning. Having Ash's friends turn against him creates an immediate source of conflict, allows the author to take Ash's character in new directions, and clears the way for him to establish relationships with characters the author likes better. The very first "Ash betrayal" story--in that it's a story about Ash having one of his friends turns on him--that I can find is Loyalties, posted January 1st, 2001. Aside from the idea of one of Ash's friends turning on him, though, it bears no resemblance to the Ash betrayal 'fics of today. Early stories in this mold usually feature either Ash's friends or Ash himself joining Team Rocket and focus more on the possibility of reconciliation, whereas the typical "Ash betrayal" story is focused on Ash taking a few dozen levels in badass and getting revenge.
I figured that if there was one story that kicked off the Ash betrayal trend, it would have to be a popular one that had attracted a lot of people interested in writing similar stories. That in mind, I pulled out the FFN data I'd scraped for my now-defunct recommendation algorithm and took a look at all stories with "ash" and "betray" in their summaries, sorted by publication date and number of reviews. The Betrayed Champion comes up as the most popular Ash betrayal 'fic in the dataset, but it was published in 2011, and I knew there were Ash betrayal stories floating around at least a couple years before that. (My data are badly out of date, too. That doesn't matter much for looking at old stuff, but does mean that the current all-time most popular Ash betrayal 'fic doesn't appear.)
After examining the top hits more closely, I ran across a story published on August 22nd, 2010, Ashura Satoshi Ketchum Atlantrome's Ace by Werewolf2300. Unfortunately, the story was recently deleted (as recently as early 2016, I believe), so I couldn't actually take a look at it to get a sense of how it played out. I did have the summary, though, which I think is pretty compelling:
Ash is betrayed by most of his friends after Wining the sinnoh leauge but lost to cynthina . Given a offer to start over in a new region, ash leaves his home town and region with his mother after being betrayed by most of his friends. AU Ash/Harem
It's pretty clear that this is a betrayal 'fic (the word's in there twice, after all), but hard to tell how much of the usual formula it includes. Is there a tournament where Ash is reunited with his former friends? I'd be willing to bet yes, but since the story is lost to the internet, there's no way to say for sure. There were a handful of similar betrayal 'fics posted shortly thereafter, which I thought might be the first wave of copycat stories. All in all Atlantrome's Ace seemed like a pretty solid candidate for the start of the subgenre.
Indeed, that's where it would have ended if I hadn't gotten curious about the origin of "Atlantrome." It certainly wasn't any place in the Pokémon world I was familiar with, but there are plenty of anime-specific locations I've never heard of. A quick Google confirmed that it isn't a canon location, but it did turn up something else interesting: namely that Werewolf2300 wasn't the only author to feature Atlantrome in their story. In fact, there were a couple of Ash-centric stories by fujin of shadows that made mention of the kingdom. And one of them, The Fallen Master, definitely debuted before Atlantrome's Ace.
I'd missed The Fallen Master during my initial search because it doesn't actually mention anything about "betrayal" directly in the summary. A quick look at the story itself, however, finds all the elements there: Ash Ketchum is betrayed by his friends, disappears from society to train, returns for a big tournament, and picks up a large number of love interests on the way. And it turns out that in addition to his association with Atlantrome, he sometimes goes by Ashura as well. Even better, the author mentions a couple of the fanfics that influenced the story: Pokemon: The Final Journey by PaulXion and SSJ04Mewtwo's Elemental Battles. These are both AshxHarem 'fics, but don't feature betrayal at all. This suggests that The Fallen Master was actually the first Ash betrayal 'fic, building on the foundation of the AshxHarem genre but introducing the conventions that would go on to define Ash betrayal 'fics thereafter. And, funnily enough, The Elementals may have inspired The Fallen Master's Ashura Satoshi Ketchum, as it uses that name for Ash. That one's been around for ages, though, at least as long as Ace Sanchez's Pokémon MASTER, so perhaps it was just a convention absorbed after reading a lot of different stories, rather than a tribute to any particular one.
Of course, given how easily I missed The Fallen Master in my initial search, it's entirely possible that there's another fanfic out there that represents the true origin of Ash betrayal 'fics. It's especially difficult to catch the-defining story with a keyword search, because the author themselves wouldn't necessarily know that "betrayal" would be a key element to convey in the summary. If such a 'fic exists, though, it can't have come much before The Fallen Master. Even in 2010, there were only six fanfics with both "Ash" and "betray" somewhere in the summary; assuming that the expansion of Ash betrayal 'fics began shortly after the first of its kind was published, it can't have shown up long before that.
Wherever they began, Ash betrayal 'fics look as though they'll be sticking around for a while--the genre didn't really start taking off until around 2011-2012 and is now well-represented among the most popular 'fics of all time on FFN. This makes it surprisingly long-lived for a fanfic trend; there have been other genres spawned off popular 'fics, like the "reborn" stories that were popular for a time after srgeman published A Little Night Music, but none of them have been as enduringly popular. The fact that Ash betrayal 'fic probably arose directly from the AshxHarem tradition offers a hint about its longevity: it combines a lot of individually-popular elements in one tropey package bound to have a broad appeal. Aside from the aforementioned shipping, betrayal 'fics often feature a stronger/smarter/darker Ash, aura powers, and ample opportunity to bash characters unpopular in the fandom, any one of which is often enough to garner a story a reasonable audience. As time goes on, no doubt there will be all manner of innovations and subversions of the genre, and perhaps in time some other trend-setting 'fic that redefines its conventions entirely, but I think we can expect Ash betrayal 'fics to stick around in one form or another for a while yet.