I like to figure out how things work, and the Void suffers from a dearth of information. So I've tried to compile my ideas and theories/things we've seen in lore in a single document. I'll probably make updated versions of this as we get more information.
Note: RITO PLS MENTION CHO'GATH IN SOMETHING I'M BEGGING YOU.
Unknowable, eldritch entities originating from the nothing. Impossibly old and completely alien. Merely meeting one’s gaze is enough to drive an Ascended to the depths of madness.
The full extent of their power is unknown, but they are capable of both producing and manipulating the corruptive, purple energy which is emanated from the Void. By controlling the nature of this energy, they can corrupt magic, convert biological matter into other forms, and presumably erase or otherwise corrupt nonliving matter if unleashed into the material plane.
Beings with innate divination abilities seem to be one of the few windows Watchers have into the world. I'm not sure if this means divination is specifically tied to them or if it merely allows watchers access to a mortal mind.
An unknown number of Watchers are imprisoned in the Howling Abyss, and the wards are failing. The Watchers are presumably the source of most Void rifts in Runeterra and the various entities and structures that emerge from them.
Outgrowths of biological matter often emanating from Void Rifts. Serve as potent carriers of Void-Energy. Largely immobile, but spread rapidly and uncontrollably, consuming all biological matter in the immediate vicinity to contribute to this growth. During the fall of Icathia, some of these structures grew vast enough to block out the sun.
Salting the earth will prevent further spread from an infected zone, but any biological matter can reactivate it and cause it to resume the growth process.
The Void Caverns beneath the Shuriman desert are home to an extensive network of Excrescence. It is unknown if these connect to a Void rift or have become self-sustaining. They presumably function like veins, carrying Void-Energy throughout. Even relatively brief proximity to these structures can prove fatal to beings without some form of protective barrier. They emerge from the ground near what was once Icathia, in a place referred to as The Rupture.
The corruptive properties of Excrescence are likely inherent to it, uncontrolled by the Watchers themselves. It is unknown if the Watchers are capable of using Excrescence to extend their own reach, or if it is capable of movement outside of its rapid growth.
These are beings that have been exposed to the energies of the Void, be it via Excrescence or proximity to a Void rift. The Void slowly unmakes these creatures, setting their forms adrift (for lack of a better word.)
As demonstrated in Where Icathia Once Stood, it erodes all sense of self the being once had, gradually removing their past and identity and replacing it with boundless aggression. It is unknown if any Void-Touched being have retained or developed sapience as a result of random changes.
Exposure to the Void is not an upgrade. Void-Touched beings gradually change until they scarcely resemble their original shape. After enough exposure, they descend into sinuous, exoskeletal creatures (see the Nami comic for examples of elderly Void-Touched sea creatures).
There are some commonalities to these changes, which can be observed near-universally in beings fully corrupted by the Void. The first is the creation of a “heart” of pure Void-Energy. This heart manifests in the abdomen, where the surrounding skeletal structure is hardened and deformed to serve as protection. The heart may be able to manifest elsewhere, but we have yet to see it.
Often, the skeleton of an afflicted being lasts the longest, resulting in Void-Touched beings maintaining the general shape in which they started for a time. Skin and muscle hardens and becomes discoloured, and teeth/mouths are typically enlarged. (Examples of young Void-Touched creatures can be seen in the Kai’Sa comic, Breach.)
Soft tissues don’t last long. They’re the first to go as fuel for the heart and for the growth of new hardened tissue. Upon parting from the source of their Void-Energy, the air often causes them excruciating pain and hardens their outer tissues into a dull, metallic armor.
Gradually, extremities are wholly unmade and armor around the heart is thickened. Non-armor tissue is broken down into fibrous structures which serve as rudimentary muscles, marionetting the body whilst allowing minimal material commitment to locomotion.
What shape a creature ends up taking likely has to do with the requirements of moving in its environment and the availability of prey creatures to provide additional mass. There is no plan, there is no rapid evolution. The changes are uncontrolled, the creatures grow and change like a cancer rather than maturing like a natural being.
It is unknown if once a Void-Touched creature hardens it can continue to grow. When there is a lack of materials, Void-Touched beings will consume one another without hesitation. It is unknown if their behavior is influenced by the will of the Watchers. It is also unknown if they are the result of the Watchers or if corruption is merely a property of the energy they produce.
Similar to Void-Touched entities, the Voidborn are the result of the mutation process being hijacked by a Watcher. Voidborn are larger, more powerful, and retain a more functional shape than Void-Touched beings.
The creation of a Voidborn requires the direct interference of a Watcher. Biological matter is used as a building block and rapidly transformed into a new shape intended to fill the unknown needs of the Watcher utilizing it.
Within moments, the afflicted creature is dead, and within minutes their body has been converted into something entirely new (as demonstrated in The Eye in the Abyss).
Voidborn have been demonstrated to possess higher-level intelligence. With the exception of Rek’Sai, all Voidborn in-game to date are sapient and capable of comprehending speech. Whether they can actually speak has been called into question. Whether this knowledge is there at birth or accumulates over time is uncertain.
The traits and shape of Voidborn wildly vary, but typically they are constructed to serve a singular purpose. The Watchers imbue Voidborn with an instinctual appetite that will cause them to naturally pursue this purpose. Vel’Koz craves knowledge, Kog’Maw is boundlessly curious, Kha’Zix desires perfect efficiency and ever-greater adaptation, Rek’Sai reproduces and claims ever-growing swathes of land.
Most Voidborn are capable of growth and change throughout their lifetime (with the notable exception of Vel’Koz and Rek’Sai. The latter may instead use growth resources to produce Xer’Sai).
The lack of direct control means Voidborn are capable of hindering the Watchers plans in order to sate their own appetites. Vel’Koz likely does this intentionally, pursuing every scrap of knowledge regardless of whether it is of value to his masters in the Void.
It is unknown if all Voidborn manifest with an intended purpose or if it develops as they grow. It is unlikely that all Voidborn are created with a grand purpose in mind (like Vel’Koz and Rek’Sai and presumably Kog’Maw). Most are likely created with the goal of killing a specific entity or removal of an obstacle. They are the hands and feet of the Watchers (or the equivalent of hands and feet for massive shapeless monstrosities). Most are expendable. When a rift opens they have more utility than Void-Touched creatures, who require a long period of mutation and a large amount of biological matter to grow into capable threats. Voidborn are already capable of taking on most threats within moments of their birth.
The nature of the "heart" of a Voidborn is still unclear, but it likely differs from their Void-Touched Kin (since Voidborn tend to retain their shape it doesn't seem to be mutating them, and some are even capable of specific evolution/adaptation.)
Only one non-Watcher entity has demonstrated an ability to manipulate the mutative properties of Void-Energy. Malzahar is capable of reconstituting biological material in the form of small, insect-like creatures known as voidlings.
Far less intimidating than other Voidborn, and seemingly incapable of growth, the strength of Voidlings lies in their numbers. Several can be created from the material found in a single corpse.
And that's all I've got thus far. Your thoughts and theories are appreciated!