Something's off about the world the Dandelions live in. While investigating Strelitzia's disappearance on Lauriam's behalf, Elrena encountered something akin to a hologram-like ghost of Strelitzia, one that, unbeknownst to her, played out Strelitzia's final moments. This hologram was a glitch in the Data World, and it's only the first of many that are about to start appearing. While it may have been benign enough, most of the ones that follow won't be.
The main events of Kingdom Hearts Union Cross quests 861 through 871 see the protagonist and Union leaders' stories finally collide. They've been running parallel to each other since the Keyblade War ended. These quests also detail the inner workings of the Data World, otherwise known as "the world after." This writing acts as an interpretation of these Kingdom Hearts Union Cross events, so — spoilers ahead.
The Plan
Ventus just wants to know what Brain is reading, as that's all he's been doing since they inherited the Foretellers chambers. Skuld wants to know, too, as she finds Brain to be more inquisitive than Ephemer, which is impressive considering Ephemer broke every rule he could to learn what he wanted. Even with both of them asking, though, Brain doesn't answer their question. Instead, he comments something cryptic: something will eventually destroy the Data World, and he's going to change that by becoming a virus and overwriting the world's future.
It's Already Begun
Ven and Skuld have no idea what Brain is talking about. However, when he departs from the duo to accomplish his plan, Skuld leaves him to it, as she knows firsthand that he's called "Brain" for a reason. That said, she's a little doubtful that his perceived threat exists, as it wouldn't make sense for Master Ava to put so much effort into helping them survive the Keyblade War if they were just going to get killed by something else. Ven agrees, and so the duo decides to discuss the threat's credibility with Ephemer when he returns.
As they're wondering what's taking Ephemer and Lauriam so long, a Data World glitch spooks them both. Hologram-like apparitions of Master Ava and the Master of Masters materialize before them and reenact the moment when the Master gave Ava his list displaying both the names of the new Union leaders and to whom among the new leaders he wanted her to give a copy of the Book of Prophecies.
Ven and Skuld can't make any of this out, though, as the glitch plays out in broken speech before vanishing. Regardless, seeing the glitch is enough to unnerve them and cause Ventus to run off and find Brain, whom Skuld says is in the tower's control room.
Meanwhile, Brain experiences the glitches, too, but not in the form of holograms. When he steps off the cogwheel elevator into the antechamber of the tower's control room, the cogs that make up the chamber begin shorting and glitching out all around him. Brain has no idea what's happening, but he knows it's tied to his prediction of the Data World ending, which he thought he'd have more time to prepare for.
Entering the Chaos
Thanks to a day of running around slaying Heartless, Ephemer has relaxed for the first time since he became a Union leader. The job's stress has kept him in a constant state of anxiety, so returning to his roots of hunting Heartless with friends has helped him let loose and have fun again. Now back in Fountain Square, Ephemer thanks the Union Cross protagonist for the good time and promises to do it again with them soon. For now, though, he has to return to his duties.
However, circumstances at the tower end up extending Ephemer's time with the protagonist. When Ephemer moves to return to the tower, he notices a giant storm of static gathering just above it. His reaction then draws the protagonist's attention to it, and so the duo decides to investigate whatever's happening together. Ephemer knows the best place for them to learn anything would be in the tower's control room, so he leads the way.
En route, the duo encounters new breeds of Heartless: Purebloods called Flappy Bugs and Emblems called Gummi Saucers. Upon making it to the control room's antechamber, they find and help a surrounded Ventus dispatch a group of Flappy bugs and then learn about the Master Ava hologram that he and Skuld witnessed. After this, the new trio works together to take down Darkling Heartless, which make a surprising return.
Ephemer then opens up the gate leading into the control room, despite Chirithy's protests. Bringing the protagonist into the control room would be going against the Foretellers' orders, and Chirithy still suffers anxiety over the idea. Ephemer has never had a problem with such things, though, and he trusts the protagonist, so he goes through with it.
The Cause of It All
Brain now has a handle on what's going on. He managed to use the control room's master terminal to stop the infestation of new Heartless — something he informs Ephemer and the others of when they reach him. He's also learned what's causing all of the chaos, but to ensure the trio understands it as well, he first reviews Master Ava's plan for all of them.
The Power Bangles that the Foretellers gave their Keyblade wielders convert darkness into light whenever a wielder fights a dark entity. This makes the darkness of the future prime for the bangles because the future contains worlds with Heartless — actual, physical embodiments of darkness. For this reason, the Foretellers used the Book of Prophecies to create reality-like illusions of the future versions of worlds that currently exist in the present.
The Foretellers knew the Keyblade War would both tear apart the real world and make the illusionary ones inaccessible. The latter was a problem because Lux can rebuild the real world, but without the Heartless, wielders won't be able to gather enough of it. Thus, each illusionary world was recreated digitally beforehand, and these virtual versions are the ones the Dandelions have been visiting. To get to these recreations, the Dandelions portal themselves from the real Daybreak Town to the digital one, and then move from the digital Daybreak to any other virtual world they want.
At first, Brain thought they were in the real Daybreak Town this whole time, but the phenomena they all experienced were glitches, which means they have to be in the virtual one. As for what's causing the glitches: a new world — one that both doesn't exist in their timeline and was never created by the Book of Prophecies — has forcibly connected itself to the virtual Daybreak Town. Brain managed to stabilize the connection, which should stop the glitches from occurring, but he doesn't know anything about this new world, including if its appearance was a part of Master Ava's plan.
As such, Brain wants to explore the world to find answers. Ephemer volunteers himself, but Brain thinks he should stay since he's the team leader. Instead, he wants to send the protagonist, a Dandelion. Ephemer and Chirithy think the idea is too dangerous, but Brain believes it should be the protagonist's decision.
The protagonist agrees to it, but Brain relents and lets Ephemer tag along after he insists. As Brain opens the portal to the new world, he informs the protagonist and Ephemer that their primary objective is to secure the route to the new world so that everyone can come and go as they please. With that understanding, Ephemer and the protagonist begin their mission.
Kingdom Hearts Union Cross is on IOS and Android.