It is hard to understand from reading I understand from what people say.
I myself did not paid much attention to the people saying it the first time I did the SL campaign. I was more interested in the story of Maldraxxus than in the general story of the shadowlands.
The second time I ran it, I noticed the people explaining the story throughout the arc, and the fact that I did not liked or interested in Ardeweald made my first run not so much productive in understanding the big picture of the Shadowlands and the powerplay that was happening.
Then there is a video in YT, old enough, and people pose that exact same question. And then I started to pay attention in my second alt to the story.
Some quotes are interesting to this:
"So the Blue eyes make sense ? "
In this presentation he lays down the reasons as to how the things happening. And he says what triggered that red thing. It is not an actual “who”.
And that is why Uther had to bypass Arthas directly to the Maw, as if it was happening after it, there would be no reason, as all souls were going to the Maw anyways.
They needed to ensure Sylvanas would survive, and they had to make Arthas serve the function Nerzul wouldnt. In order for the things that happened, happen.
Then you have the things orchestrated to a specific situation that was what triggered an specific soul to “glitch the Arbiter”, for an specific reason. Zovaal knew this because he knowns how the Arbiter works.
The problem is understanding the timeline of events. People are saying things basing the idea that this plan started in this xpac, and they simply “created” shadowlands plot now.
As I said before in other topic: