The void is many things, yet most notoriously of all it is nothing. It is the pre-existential nothingness from which all was created and to which all shall return. The void is beyond time and space, existing both outside of the universe entirely and between every gap between every sub-atomic particle. The void penetrates everything, destroys it, changes it, and leaves the infinite potential for new creation to occur. As such, the void is deeply linked with entropy.
Understanding the void can be beneficial to any practitioner who wishes to change themselves and the world around them, but working with it too deeply can be very dangerous even to advanced practitioners as it is a very fast road to occult madness. Don't mess with it unless you really feel called to, and always take a guide with you who knows their way around the timeless spaceless paradox of existent non-existence.
The void consists of various layers, although these layers are less defined than those of the planar realms. They're more like different flavours that your brain synesthetically translates into colours, forming endless fractals. Within the Qlippoth the void takes the form of Da'ath, the abyss that must be crossed before the practitioner can descend into the final three qlippa. Da'ath permiates both the tree of life and the tree of death and is but a different expression of the same fenomena in both cases, even more obviously so than any of the other spheres, as it is not truly a sphere to begin with, but rather the void between them.
Beyond Thaumiel the void takes the form of three more layers. Tohu, Bohu, and Chasek, but we will not go too deep into those right now, as I still don't know enough about them myself. These layers however are also mirrored on the tree of life as Ain (Soph (Aur)), as source is both the antithesis of the void, and simultaneously the exact same thing.
The void is often said to function as a hivemind, especially when talking about Da'ath, and though I have still to fully connect with it, there does seem to be such a thing.
The void however, exists not only within the contemporary western left hand path, but accross many cultures and religions. Something, must always come from nothing. Often times it is compared to primordial oceans, mist, or simply chaos, and I would personally say that while the void is all those things, it also predates them, as an even emptier infinity. Though, at this point we can not speak of "predating" in any sort of linear time sense.
When working with the void, you will become more like it, as you will practicing with any current, deity, or realm to be honest, but especially with the void it can make you lose touch with your humanity. And yet, to advance with it, you do have to become like it, Solve et Coagula. Destroy and create again, with limitless possibility. One must learn to be a part of it without being consumed by it entirely.
The void is home to some, and scary to others.