"In the outskirts of the city, the Spirit Wilds have been segregated away from the normal populace. Spirits have been treated as second class citizens compared to their human counterparts. When the portals were opened, the desire was for humans and spirits to live in peace but it seems that wasn't possible.
With the coming of the spirits in the form of dark versions of themselves that terrorized humanity, people continued to fear the possibility of the return of that darkness. Many feared the possible return of a Dark Avatar; a reincarnation of Unalaq and Vaatu after they merged. Others argued that such a thing was possible, but no one knows for sure.
Since the time of Korra, it seemed that such a conflict with the spirits wouldn't happen again. The darkness appeared to be quelled and no Dark Avatar appeared. Soon, the people stopped their murmurs of such a possibility and the horrors of those days were nearly forgotten.
However, in the time of Avatar Renshu, dark spirits began to appear once again but this time relegated into the Spirit Wilds. No one was certain of why. Anyone who ventured into the wilds either fled for their lives or ended up missing and presumed dead. Those who escaped heard the spirits mention an entity called "Mother". What this Mother is for certain still remains unknown.
It wasn't until the members of Catalyst followed a man into the wilds, believing he could somehow bend multiple elements, was at least some of the truth discovered. They eventually confronted this man and another person identical to him. To their surprise, the man they followed changed form and became a large dark entity that appeared to be composed of the four elements.
This being divulged parts of its existence to them as well as it's plans. It had been creating dark spirits and forcing them to permanently bond with the humans that were captured in the wilds. Through this process, a being very similar to an Avatar was created, but to a much lesser degree. They were able to take on spiritual forms as well as remain control of their bending. However, their deaths did not result in a reincarnation.
It wasn't long after their meeting that the Avatar himself showed up to confront this being as it had finally revealed itself. He had been privy to its existence but even he couldn't pinpoint its location as it had been hiding in the form of the man that Catalyst was suspicious of.
Before his arrival, the being had taken on a purely human form and reveal that it was not a spirit as they had thought. Once the Avatar did arrive, he muttered things about his past and about a woman named Cera. He appeared to have some connection to this woman.
Very shortly after, the woman and the spirit-humans under her control vanished into the darkness and left only one clue. The Avatar was the one who deduced they were heading to the Wastes for some reason. At this point he interrogated the members of Catalyst before deciding to follow this woman to the Wastes. Catalyst themselves also decided to go to the Wastes."
I write this message to you all this message not only to have you be weary of the times to come, but also to stir you up in defense of this world. There is no doubt that this Mother is a threat to our world. I plan to join the fight in the Wastes and help save this world.
The Avatar cannot do it alone and I hope you all feel the same. I will likely lose my job for posting this article without permissions but this is bigger than myself. If you wish to join the fight with me and the others, there are many ways you can get there. Many of us trying to find a way to get there and just as many helping others find their way. If you really want to find a way, you need only ask the right people the right questions.