OK, so there’s a place in Revendreth called Spire of the Unseen Guests.
Unseen Guests is how the Dreadlords call themselves.
In it there’s a book, written by Dreadlords (that was confirmed in a Blizzconline interview) to the guy that “sired” them, heavily implied to be Sire Denathrius.
To our most wondrous and resplendent Master,
I am pleased to state that, after a lengthy study of our rivals, I have completed my observations.
Please accept this briefing in advance of my complete report, which shall follow forthwith.
We anticipate that certain of our targets will be more challenging to topple than others. But each is prone to manipulation in different ways, and our agents have already woven themselves into their very fabric.
This is, after all, the solemn duty for which you sired us. As you are fond of saying: Once a desire is understood, it can be exploited.
Enough prologue. Allow me to summarize our findings.
In many ways, the titans will be the easiest to manipulate. Their singular goal is to impose structure upon everything they see.
Show them a force that opposes their drive for Order, and they will be consumed by their urge to eradicate it.
Their pantheon, so seemingly united in purpose, is vulnerable to fracturing.
The void lords all but welcome us with open arms. They are so preoccupied with their thousand truths that they ignore the lies we sow in their very midst.
I believe we can leverage their vast reach to position them as a foil against our other rivals.
We remain wary, though. Since they are observant of multiple outcomes, it is conceivable they could anticipate our coming.
Similar to the titans, the naaru and their keepers are singular in purpose. Their adherence to a linear path is an obvious shortcoming.
They savor nothing more than being proved right, so if they believe they have converted one of us to their precious Light, they will trust that agent implicitly.
The adherents to Life are the most insidious of opponents, perhaps because their nature is so antithetical to our own.
Still, we learned much from observing the link between their plane and Ardenweald, and we have high confidence that a vulnerability has been identified.
Our operative has already gained the trust of her target.
And as previously discussed, our position within the plane of Disorder is proceeding flawlessly. Consuming fel energy is not a pleasant process, but a necessary one.
The deception you have architected will bear fruit in the ages to come.
As ever, we shall serve as your unseen hand. We will poison every host foolish enough to invite us into their midst.
I remain, as always, your faithful servant.
Nathreza was a world where they lived in the Nether as they infiltrated the Legion, but they weren’t from there. They are from Revendreth.
Another interview on Blizzconline also confirmed that they are returning to the Shadowlands in 9.1. They are also very likely to be the people that got the Helm of Domination and Frostmourne in the hands of Kil’jaeden.