Elections year

The conflict

At first, most people didn't know they were sick, and others were diagnosed of different mental disorders or neurological diseases and didn't know something worse was occurring.
As they lived their life as normal, a plague was invading The Surface, and misremembering small details turned into a whole alternate reality building in their minds.
Even on elections day (the date many associate with the disaster, things were just fine. In every city at the south west of the continent (and a few on the north-west), people showed up at the nearest park to vote as they had done every five years since they had turned nineteen. They draw a square around the labeled picture of their favorite candidates for president, mayor and local High Judge. Many of them had a nice talk with their neighbors, regardless of their political differences.
There weren't any issues when, a week later, it was announced that the new president of the nation was Eaminá, an elder from The Healing Order , born at a small farming city in the south west but currently living in a northern temple.
When she was contacted by her friends, relatives, and political party to congratulate her and to organize things, she was surprised. She has never been a candidate because she was en expert in medicine, not leadership or administration. And, of course, there was the little detail that there was no democracy in The Surface. Every city and temple had always been a meritocracy to some extent, and the closer thing to an election was when the members of the General Government voted because a discussion had lasted too long without consensus.
Sociologists were the first to analyze the situation, but soon the psycologists and neurologist were involved, as it became evident that this wasn't a political issue like the manifestations and demands from the west seemed to be. They honestly believed that there had always been elections in their world.
They would be confused when they saw the blank sheets on which they had draw a squared, or the history records detailing the results of the tests of the current Manager of their city. They would ask questions, trying to find a trick or to understand why they remembered something different. Some would get angry, but many would be confused or scared on the face of the evidence that didn't fit their memories.
Disregarless of their reaction, it didn't last. Not a minute later they had forgotten about their shock and any explanation or argument on the subject. "Oh, yes, I did signed that vote, it's all in order. I've never heard of the other guy, he might be nice, but Eaminá is a farmer and a cleric," some would say, perfectly happy. The records? They didn't remember exactly but there was a couple of familiar names from the new government, right?
The first theory was that they were hallucinating due to a chemical, or some form of magic, but soon they understood that they weren't perceiving things wrong, they were remembering events that had never taken place, and conveniently forgetting anything that didn't fit with those imaginary facts.
They accepted to be seen by medics and many of them would admit that they had felt a little confused about little things, but none of the would remember anything that went against the results of the elections. But they all noticed the absense of Eaminá.
While everyone else was worring about their health and memory, they were starting to remember that the righful ruler was being held hostage in a temple to stop her from taking power, and that they were all part of a plan to rescue her.
Yes, whatever form of disease or attack that was, their shared memories included a clever organization to fight several neighbor cities.
By the time the North temple was invaded to "rescue the president", many scientists and medics (clerics, they call them) were already infected by the memory changing disease. The people that tried to help or stop them on their way to the temple were showing symptoms too.
In a way, that was a blessing. Someone carried the sickness into the temple in time for it to develop, wich prevented the loss of lives in a pointless fight. But the proclaimed president had escaped the area and now her "followers" were preparing to fight the rest of the world... or at least, those that the disease didn't changed first.
Several battles took place during the next years, and disregardless of the results, people of the area would start to change their minds after the event.
Meanwhile, Eaminá wasn't just hiding. She and one of her most promising students designed a way to observe the infected without any kind of contact with them, and to conduct a research to understand and cure the disease. The pupil, eydeamt, contacted a few genious of other areas to design a type of drone to observe people in the affected regions, in hopes to record enough information about how the disease spread. While Surface' inhabitants have impressive AI systems, the code on this drone was especifically designed to observe without thinking: recording only facts and unable to edit or analyze any data. Due to the constant movement and violent behavior of the people infected, the drones couldn't collect much information from them, but they recovered some of the original data from previous studies. They were also used to to mantain the communication with some clerics working in the infected region; which was pretty sad, because they started as researchers and ended as subjects, while their friends from other places sawt them loose their mind and wondered if they would be next.
All the new knowledge only pushed the remaining researchers into despair. And that's where we enter.

The wish

The most determined and empathetic of the clerics found herself in our building after a bad night of sleep. She didn't trust our means and went back home. Several times. But the more time passed, the less options she had, and more people got sick or hurt in battles that nobody had really started. Eventually Eydeamt wished for a cure.
She was given a myth, an incomplete map, and a visit to a flower's field. I told the... boss that that was stupid. And I'm sure that the Selection department thought the same. A whole civilization was about to dissapear, and the granter's rules were making the task to save them close to impossible. Nobody had ever found the healing food of the story, and it didn't take any effort for the granter to simply made it appear in front of Aye if it really existed. They hadn't done anything to deserve what was happening to them, so why make it so hard?
But of course, the current rules aren't for punishment of fairness. The boss just don't give free stuff, he build a business that sells one opportunity. She could have asked for answers, or the tool to find the origin of the sickness. Those would have been tools for them to develop a cure. But the whole homework finished and polished? Not what we do here.
I do like that, and the rules, but that one time, it was nonsense to me. I would have take her side if Aye had asked for something better or simply yelled at him and leave, again. But she didn't only accepted the task, but completed it... almost. She found the cure in a haunted house... and didn't have the heart to steal it. You see, the price was to high, not for her or her people but... the Surface' theists value life too much, you know?. That's what made the whole "fight for democracy" less horrible in general terms, but also abominable to their eyes. And that's what stopped from taking the cure to her people.
Instead, Aye stole a powerful magic tool, with the hope of using it's magic to heal them, or at least to stop the violence that even their beliefs wouldn't minimize forever.
Once again I thought it was a stupid idea, because she was taking an unmanageable weapon to do the job of a precise medical instrument, in a world that was already in the middle of chaos. And, once again, I was wrong.

The Disseverment

Aye did break The Surface in two, just as I had predicted, but it wasn't because she lost control of the weapon she stole from us. She did it because that was the less dangerous way to use it for her goal.
It was the last step of a plan to keep everyone safe, sane if possible.
First, Eaminá assumed the role that has been chosen for her. That way, her followers wouldn't dedicate their lives to fight against imaginary enemies. If she was their president, she would do what clerics always do: work hard to protect any life that had been trusted her. She planned to rule from the border, trying to keep her sanity for as long as she could, and when people heard about it, many volunteer to join her. They offered different skills to support her or to help with her job.
They settled somewhere near the affected cities (where they would build the Temple of the Memory) and reported back to Aye, who casted a single spell with her recently aquired magical staff. A spell meant to create an umbreakable division between those affected by the disease (including their president) and the rest of The Surface.
I'm pretty sure she was hoping to build a wall or a mountain. They had even relocated people from the area. But nothing happened. I heard she broke in tears and all.
She meant to try again as soon as possible, but before she managed to understand what had failed, people started moving out of the cities close to where they had expected to have the division. Many to the infected cities, and their neighbors would see that and though it was sad, but never thought of asking them to come to the healthy region.
Those who weren't traveling, wanted to do it but didn't for some reason or another. They practically ran a few weeks after, most of them because of the seemingly innocuos sprouts growing everywhere: they couldn't stand to see them, but they didn't dare to remove them either.
On the side that was expected to be "on the other side of the wall", sprouts appeared too, but didn't invade the cities and not everyone flee. Not even those who stayed would dare to enter the forest that grew there over the next decades. Even animals escaped it.
The trees grew through houses, mountains and even rivers, turning everything into... soil, I guess. An extense and varied area of The Surface was turned into a valley, where all you can see now, is beautiful gray trees.

Notable individuals involved

President Eaminá

Eydeamt

The Surface

Currently, both sides of the world live their own reality and... last time we heard about the Democratic Surface they were doing alright.
Right in the middle, somewhere in the Valley of Disagreement, the Temple of Memory is inhabited for some healthy people who keeps communication through the brainless drones.