I can post what is written in the Bestiary (GoM edition):
Noth. A meaningless name for something that should not exist. These entities can not even be called monsters or nightmares; things like good and evil are irrelevant for entelechies that simply disbelieve everything in their path. In more than one sense, they could be considered the end of everything that exists.
The Noth are foreign entities that are impossible to understand or explain. The only thing that can be theorized about them is that they are devoid of concepts or something like a "nature", so they can not be linked to anything. And that is precisely what they are; absolutely nothing. They can't even be considered void elementals, as even beings like the Nosos Etrien are linked conceptually to the structure of existence. The Noth, however, do not respond to any rule or principle, as if they were strangers to absolutely everything.
The Noth are an almost unstoppable divine force. They manifest through vortices in different unrelated places and swarm aimlessly for a short period of time (never more than half an hour). They perceive the presence of things around them and approach those with a higher existential potential. Then they enter in contact with whatever has drawn their attention and consume it. There is no difference to them if it is an inanimate object or a living being; when someone tries to escape, the Noth just shred him until he dies or is unable to move. If they are attacked, the Noth respond showing clear signs of possessing disproportionate combat capabilities; even the weakest of them are creatures of aberrant power. Their appearance is difficult to explain. Physically there are many types and sizes, but their forms are always consistent with each other. All of them are completely white and their entire bodies seem to be made of a single substance, like a uniform mass that has different form. They also don't have internal organs; the Noth are completely compact. Interestingly, the surface of their skin is incapable of being stained in any way, so they retain their characteristic whitish color at all times. The Noth also usually have some kind of spiritual cord on some part of their body (usually in the area equivalent to their necks), which disappears after a few meters in some kind of existential vortex that moves with them.
Probably the appearance in which people perceive the Noth's are not even their true form (if they actually have one); it's just the way that the mind of those who contemplate them interprets what they are. Thus, two people who see the same Noth can agree in its general appearance, such as saying that it is a milky colored humanoid, but both would see it in a slightly different form. Noth are invisible to the human eye and can only be perceived by individuals able to see supernatural things or have a higher Gnosis (the higher it is above their Natura, the clearer they can see the Noth).
A Noth never communicates nor seem able to understand anything that surrounds it. In fact, only some of them (those who are considered superior ones), seem to have glimmers of intelligence. However, they never act in an organized manner; it is as if they are not even aware that there are others of them nearby.
The Noth don't generate existential energy nor can their skills be detected by magical, Ki or psychic abilities. They can still be affected by such powers, although it is completely impossible to alter their will or mind. For all purposes, it is as if they simply did not exist.
There's more in the Bestiary and in Gaia Vol II (Although this one isn't translated to english) but I do not ant to fill the thread with large amounts of text.
