Spoilers
Kingdom Hearts Union X is ending on Friday April 30 so let’s put forth Last Minute Theories here:
I suspect that the Darkness absorbed Ventus’s Sin resulting in the Darklings becoming Nightmares over time and the fragment of Darkness that split off rectified it by shoving the Sin into Spirit Dream Eaters causing the ones belonging to those who fall to Darkness to become Nightmares themselves though it may likely only be revealed in the next KH Game.
In all likelihood once the Darkness that split off from the Darkness that was turned into Vanitas is killed all the Darkness will become Wrath(since that is the Sin that Darkness seems connected to) and the Heartless will become Nightmare.
Furthermore I suspect Ventus’s Clash with Darkness will create the X-blade and Ventus will bring it to the outside world in hopes of destroying the Darkness only resulting in the Unions fighting over it.
I suspect the X-Blade’s destruction will signify such victory over Darkness that Kingdom Hearts will counteract the imbalance by creating a Flood of Darkness just like it was going to counteract Xehanort’s imbalance of Darkness with a Flood of Light.
The Hearts of the Children will provide enough Light to set up a new day yet first Ava’s Book of Prophecies will enter the Black Box which will use it to create the Worlds first by burning away the Darkness with Fire causing Kingdom Hearts to react to the Imbalance by creating a Flood of Water then the Book of Prophecies will manifest the Worlds of Kingdom Hearts on top of the Water.
Now if an imbalance of Darkness creates a Flood of Light, imbalance of Light creates a Flood of Darkness and an imbalance of Fire creates a Flood of Water then what does an imbalance of Wrath create? What is the opposite of sheer Rage? Peace? Happiness? Nothingness!?!
The last one of course would result in everything being smashed by Nothingness into the Non-Existent World of Quadratum merging the two Worlds so that one seems likeliest!
So what are your theories?