Little is known specifically about how exactly the gibbering mouther first came to be, but the earliest records of the creature definitively point towards Limbo, with numerous sightings over the course of its first centuries of existence. It was not for many centuries after the first known recorded sighting before the creature was noted as appearing on the Prime, though whether its first entry there was accident or purposeful is lost to the mists of times.
While the precise origins of how the mouther came to be are unknown, their nature combined with their own natural corrosive properties lead most scholars to think of their initial spawn as the result of a singular manifestation of the corrosive effects of Limbo's mass on a traveling party on that plane, breaking down form while preserving substance and merging the individuals into a single disgusting mass of nondifferentiated and insane flesh.
Given the utter madness of a gibbering mouther and the solitary life they hold, they have no culture to speak of. Two gibbering mouthers, when brought near one another, seem no more cognizant of one another than of any other object.
As nearly-undifferentiated masses of living tissue, a gibbering mouther has no internal organs, no circulatory or respitory system, and nearly no nervous system. While it does possess a mind, this mind is wholly distributed throughout the mouther's form, with no singular brain or equivalent. While they do have innumerous mouths across their form, and while their vast ability to digest nearly anything is well-known, neither do they have any digestive system; instead, just a series of temporary sacs that connect to their gnashing maws.
Only a few things are immune from their digestion, including glass, diamond, collagen, enamel, platinum, adamantium, and mithril. This is in large part thanks to their natural corrosive properties, a natural ability rather than an alchemical one that in many ways is similar to that of a menglis if far more slow-acting. However, it's aided by their bodily fluids, a formulation similar in form to the extremely explosive distillation of brine and spirit of hartshorn; this substance, in addition to serving as a defense that can be spat against predators, literally breaks apart the largest pieces consumed in order to allow them to more quickly be decomposed.
Most indigestible materials are eventually expelled from a mouther's body, though the corneas and teeth of victims are held, used as based around which the eyes and mouths of a mouther are formed. Similarly, the very thoughts of any living victims, regardless of nature, are also used by the mouther, incorporeated piecemeal into the gestalt of its thoughts and occasionally surfacing in fragments in the mouther's pained ramblings. It's unknown, however, how this incorporation impacts the spirits of those killed by a gibbering mouther, as there are no reliable records of attempts of resurrections of the victims of a mouther, successful or failed.
While a mouther can survive on a purely-mineral diet, it requires organic material in order to reproduce. Upon consuming enough such material, a gibbering mouther eventually reaches sufficient size to divide in two, with its corneas, teeth, and gestalt divided evenly between the two offspring. The rate of reproduction seems to be entirely based on the consumption of food with age an irrelevant factor; in fact, all tests suggest that a mouther can live eternally even when deprived of sustenance.
Gibbering mouthers appear as near-liquid masses of flesh of all colors and hues, similar in appearance to the morass of Limbo, their flesh dotted with the signature eyes and mouths that are constantly emerging and collapsing into its form. They feel as slick massed of half-rotted flesh with the tingle of their natural corrosion, and move much as massive amoebae via the extension of pseudopods that allow them to flow along surfaces.
The most significant feature of a mouther, however, is the gibbering that gives them their name. A constant flow from the gestalt that is a mouther's mind, the gibbering is nothing more than the expression of fragmentary thoughts that float about via the victims they consume, repeating and recombining almost as though the mouther is trying to find some sense in its existence to no avail. Indeed, many describe a thick sense of remorse dripping from its words, although others do claim instead a sense of hatred or anger, an effusive ecstasy, or other varied emotions across the entire spectrum. Whatever the nature of the gibbering is to a mouther itself, though, something about the meaningless fragments resonates with the minds of other intelligent beings, bringing them to the same insanity that the mouther itself suffers from, if temporarily.
- Dragon #160 - Ecology of the Gibbering Mouther, pp.55-57
- Monster Manual, pg.104