The Infinite Madness, also known by many other names of which there are far too many to list, is the origin of all Eldritch creatures and concepts, and the ultimate example of Cosmic horror. It is a dire threat to not only the entire Barrelplex, but very possibly even far beyond it.
As far as it is currently known so far, nothing could ever stand a chance against the collective whole of the Infinite Madness. Of course, given the infinite nature of The Box, there are many things which are/were near-equal to The Infinite Madness(Such as Eternal Defiance, though in this case Eternal Defiance is currently nowhere near powerful enough for such a thing). There also exist an unfathomably infinite number of things far more powerful than The Infinite Madness, some of which utilizing it as a tool themselves. However, nothing is known about any of these forces, thus The Infinite Madness is usually considered to technically be the most powerful thing besides The True God itself, even if this is not strictly true.
There is practically nothing that can be truly confirmed about the Infinite Madness, other than basic principles such as it "is and is not". Because knowing anything about it, even knowing the concept of it as a potential entity, has the potential to cause violent mutation and insanity within beings who are ill-equipped to properly understand and comprehend its nature.
There is no possible origin to the Infinite Madness, as it had never been "created" in the first place, and no external forces were involved with its manifestation into (un)existence. It is simply a pure, unbridled cosmic force of nature, driven only by its primal instincts. One of the few things known for sure, is that it and its spawn spread their influence from verse to verse, dimension to dimension, and leave naught but chaos, destruction, madness and terror in their wake.
How the forms work
Contrary to popular belief, there are no separate consciousnesses of The Madness, but instead it is capable of manifesting itself all throughout The Box (Except for places in which it is specified that the Madness can not access, though it is currently working on a way to bypass this), an infinite number of minor manifestations of a larger, more sinister force; like the tips of multiple icebergs breaking the surface, only for them to all actually be the edges of one incredibly massive iceberg beneath the water.
Essentially, The infinite madness is a Hivemind, with its individual manifestations/avatars acting on its will.
Avatars - The Unthinkable
"The Unthinkable" are the avatars of the Infinite Madness, with an average one equal in power to the most powerful of Aspect Lords. The Unthinkable are agents of chaos and insanity, driven by a desire to destroy all forms of structure and plunge all possible and impossible cosmologies into utter anarchy. Although an individual Unthinkable may seem unique in its behavior and mannerisms, whatever free will it may seem to possess is merely an illusion. It is completely impossible for any individual Unthinkable to resist the collective will Infinite Madness, as they are all part of the same entity. And in the end, no matter what, every single action made by an Unthinkable is on the behalf of the whole of the Infinite Madness. As the Infinite Madness is, of course, Infinite, every single Unthinkable and its identity and circumstances are completely unique to the individual, and no two are ever similar.
While "killing" or "destroying" an Unthinkable is a great and quite notable accomplishment, it is ultimately meaningless in the grand scheme of things. The death of an Unthinkable is akin to a single leaf falling off a much greater tree. At best, it only serves as a minor inconvenience to the whole of the Infinite Madness, which can simply and easily create a whole new Unthinkable to replace the lost one and continue work where the previous one had left off. Such is the tireless and persevering nature of The Infinite Madness.
The Rulebreaker currently works alongside an Unthinkable to earn the whole Infinite Madness' favor and gain power against his enemies, since Aspect Lords are always constantly fighting many forms of the madness and so Rulebreaker sees The Infinite Madness as the perfect weapon to use on them.