What It Is
There are no names for it in any language because no language was made in its presence. The closest any tradition comes is a negative: the Unwitnessed. The thing that exists without being observed -- which is a specific abomination, given that existence was created by an act of observation.
The Unwitnessed does not come from the Null Void. The Null Void is nothing, which is a defined and containable concept. The Unwitnessed comes from what the Null Void was built to contain. Before the Observer. Before the concept of before. Something the universe was constructed, unconsciously, as a wall against. It has been pressing against that wall since the first second of creation.
How It Manifests
The Unwitnessed has no form. Every mind that perceives it does not see it directly -- the mind places over it whatever that specific consciousness finds most intolerable. Not most frightening. Most intolerable, which is a different and more precise category. For Aevun, it appears as a point in The Chronicle where the timeline ends with the quality of a story erased. For Soryn, as silence -- every mind in The Resonance going dark at once. For Keth, as matter losing the property of being matter.
In the mortal world, the three places it has touched the wall manifest as zones of specific wrongness: places where the rules of reality hesitate before applying, where time is inconsistent, where space is unreliable, and where those nearby experience the sensation of being on the edge of a thought they cannot complete -- an almost-perception that leaves an empty space where the perception should have arrived.
The Relationship with the Powers -- Why Fighting Attracts It
The Unwitnessed is attracted to the powers of the Seven. Each power is a beacon of concentrated cosmic energy, and the Unwitnessed orients toward it the way things that have never known light are drawn to light. Every time a Primordial uses their power in response to the Unwitnessed, it moves closer, drawn by the exact effort to address it.
The only path forward may require each Primordial to suppress their power entirely -- to become, for the duration of whatever confrontation is coming, simply themselves without The Chronicle, without The Resonance, without any of the six remaining powers. What a Primordial is without their power is a question none of them have ever had to answer. The novel's third act is the answer.