"What you cannot grasp nor acknowledge is what makes us who we are, but does that make us dangerous?"
(TL;DR at bottom)
The Unfathomable (aka "Fathom" both singular and plural) is a Living Verse that acts as the power source to all cosmologies, that pretends to allow Law and Reality to co-exist making dimensions possible, to function the way they intend to. In laymens terms, it's a Verse that allows everything else to exist and function, at least according to theory, nothing is set in stone with this. Within exists creatures dubbed the Paradoxant or otherwise known as Fathom as a take of it's Verse.
However this Verse is not one someone would voluntarily want to live in nor even visit for being exposed to it could cause someone to be infected with what is known as Jecht, a sort of "virus" if you will that warps someone's very constructional being and how they work, almost putting them into their own state of physics independent from their own current dimension they reside in throwing reality around them into a state of dissaray that lasts usually temporarily, but can be permanent.
The purpose of The Unfathomable is to infuse everything together into a perfect Nexus of sorts, almost like a sense of Nirvana. The downside to this is that during the process horrific things can happen such as the stages one is required to go through in order to obtain this state such as horric fusions, mutations, and other sorts of unorthodox effects towards ones very being.
It has been noted that The Unfathomable does not conform to our laws nor does it desire to as it can decide for example that colors can be numbers and alphabetical letters can be geographical directions. Nothing has to make sense due to it's nature, however those who are lucky enough to make sense of it can live within this Verse as a separate entity from The Unfathomable altogether. These beings must have a God tier level or higher to not be effected (this varies the deeper they go to the point where the deepest point of The Unfathomable requires you to have a tier level of the The All-Creator) by the depths of it's void.
While it's purpose is to combine everything into one, it's goal is actually quite different. The goal of The Unfathomable is to erase The Cycle of Everything (not to be confused with Law of Dimensional Cycle) as that keeps Reality safe from The Unfathomable by repeating absolutely everything so it will never reach a point of freeing The Unfathomable onto all cosmologies. Some say this is for the best as resetting everything keeps Reality safe, but at the sametime some say it's bad as it keeps us from expanding beyond The Apex of what we can learn, do, and see. In either case, the only thing that can break this cycle are either the Other Philosophers or The Wielders.
(images and videos from the movie "Annihilation". They merely show a mild idea of what The Unfathomable do.)
The infamous theory roaming about that surrounds The Unfathomable is the idea that they have already won in a sense, that they have already reached "Every" in their Nirvana state where everything, everywhere, and everyone has become one entity/Verse. The reason for this theory is that The Unfathomable is for the idea of constantly making new things as well as evolving them. This would explain "De Ja Vu" which is the sensation of already experiencing this thus we would act on it with more caution and sometimes even more curiousness leading to the newer universe being more advanced than the other if even by a little, then the next one is more so, then more so, and so on and so fourth.
The only other reason this theory as infamous as it is for it's implication can exist is due to how The Unfathomable act- they are infact undetectable unless you are set to a certain layer of Reality. As in they have absolutely no trace left behind almost as if they are above the Law entirely. The good news is they can only be physical (for now) by planting ideas in others minds through the Jecht method and make others act upon their will in order to one day free them before The Cycle of Everything repeats itself and resets Reality. Some say this is merely the infamous theory working to keep others thinking they won while the more welcomed idea says this keeps them from winning no matter how close they get. After all, evil never truly wins... Right?
Other Theories: 1. The Unfathomable derive from Anti-Matter that did not die during the Big Bang, but rather it disappeared from our world and formed another Universe where it grew from there. 2. The Unfathomable are merely past incarnations of Existence. 3. The Unfathomable is a corrupt Cosmology made by an apathetic Philosopher.
(Author's Note: some common misconceptions are that The Unfathomable accidentally trigger The Cycle of Everything, but this is not true, they merely bend everything back to the "Every" Point so that they can evolve life in the next cycle, while The Cycle of Everything does not evolve life in the next cycle in order to stop lesser beings from becoming too strong.
The other misconception is that The Unfathomable while being the power source to all of Reality it does not mean they were originally supposed to be alive. This was instead caused by an oversight not seen by the Other Philosophers that is too late to correct.