Etherkai, the Nightmare Dragon is the first in a series of "Worldbreakers," enemies that are so powerful that they can actually change the terms or conditions of the encounter or even literally affect the terrain. Written as modular sorts of enemies, they are designed to be able to be easily dropped in to any type of adventure with minimal or no re-skinning. That being said, as I was reading Etherkai for the first time I knew, instantly, exactly where I would use it in the forthcoming campaign, without any modification whatsoever.
The background text, which constitutes the first few pages of the "book," is very well-writen, evocative, and interesting. I don't know what else to say about it without giving details away, but the story of Etherkai, and how he became what he is, is very complex and even a little bit sad, in a way. What is clear is that Etherkai is very frightening, compelling, and obviously needs to go in a major way.
Etherkai's stat block constitutes a whole page of content, as does his worldbreaking effect when it comes into play. As a level 10 monster, Etherkai is perfectly fit for a culminating encounter at the end of the Heroic tier, and he is powerful enough that it will likely have the overall effect of a "boss battle," even if Etherkai is not, in fact, the "big bad" of the tier. Perhaps the dragon is being manipulated by an even stronger power; perhaps he is just an accidental obstacle that turns out to be far more dangerous than anticipated. But I think regardless, the strength of the worldbreaker is that, since they have the ability to change the setting of the encounter, they act as a liminal space between one kind of storytelling and another and provide a literal, as well as a metaphorical, break between two arcs.
I look forward to future releases, as the author, Quinn Murphy, has proven himself a very capable and creative writer, and I really have nothing bad to say about this invention. I only wish I could run the monster sooner.