For your brain devouring monster needs.

Encephalids are horrific eyeless creatures with heads that appear to be nothing more than an exposed, over-sized brain, on an apparently humanoid body. The flesh is pale, near albino, as if from many generations living far below ground, and in fact when they choose to live openly, they almost always do so deep underground or deep underwater. There are rumors and legends of great cities of them in both locales, far from the touch of sunlight. They are fully amphibious. Their flesh is also rubbery, and slimy, like the flesh of a snail or an octopus. They seem, in fact, to have no internal skeleton, or perhaps an extraordinarily malleable and compressible one, formed of some sort of cartilaginous equivalent. When needed, especially in combat, the creature extrudes multiple tentacular pseudopods from its body to use in attack and defense, each bearing a hollow stinger at the end that is used to siphon off bodily fluids from their prey, especially blood and cerebrospinal fluid. They also possess large lamprey like mouths in the center of their radial, ten digit 'hands', that rest on the end of the larger and more muscular primary arms. These mouths are used to ingest solid flesh, especially brain matter, from their prey species. The radial digits clamp down on the skull, squeezing it with enormous force, until a portion of it in the middle cracks and protrudes, and the lamprey like mouths extend on a radula to pierce the skull and extract the brain matter within.

Encephalids are sometimes believed to be some sort of hideous mollusk, and in fact there are resemblances, but they are actually a hybrid organism, a merger of a parasitical, nearly formless colony organism with a host. The host of the creature we know as an encephalid is a humanoid, of any sapient kind, but any terrestrial organism seems to be able to host the parasite, with extreme differences presenting in the results depending upon the host species.

Perhaps the most horrific thing about encephalids is that when they parasitize a sapient host, they gain all of that host’s intelligence and memory, perhaps even increasing it significantly during the transformation. They also seem to gain most of the knowledge and memory stored in every brain they devour. And they seem, as well, to have abhorrent long term plans, often beyond the comprehension of normal intellects.

In their larval form, encephalids are tiny, near-invisible, plankton-like creatures that group together in colonies. The larger the colony, the greater the inherent intelligence of the whole, but they seem unable to survive for long outside of a host, unless they are in a fairly specific liquid environment, greatly resembling amniotic fluid. To survive under the conditions of our world, which are hostile to them, they must parasitize and inhabit terrestrial hosts. We are not aware, at this time, what sort of creature they might be, or become, in their home environment...which we likewise know nothing of. They do seem to come here from some other world...but where that world is, and what its nature might be, we are unclear on. It may well be that very different physical laws apply there, implying that it is either a very alien locale indeed, perhaps even extra-planar, or as some sages have postulated, even extra-multiversal.

Terrifyingly, the creatures possess an ability to manipulate their physical forms that resembles the abilities of octopodes and the like...but far more versatile. Each tiny part of them seems to be able to shift in color and consistency, and even in orientation to the other bits, allowing them to blend completely into the background of their surroundings, become transparent and nearly invisible, or even to fully shift into a shape that mimics that of a normal humanoid, and sometimes even other creatures. It is easiest for them to take the form of a being whose brain they have ingested, perhaps using the memories ( or even somehow the very nature ) of the creature  as a pattern for duplication, but they can also take more generic forms, and devise new forms of their own. In addition, like an octopus, they can compact their form down to squirm through the tiniest of openings, even their mouths compacting, lacking the octopus' beak that limits their reduction in size. They even seem to be able to manifest a glow from their flesh like some fungi and other creatures do.

Their over-sized and extremely advanced brains grant them enormous mental powers, most commonly telepathic ones, used for information gathering, communication, defense, and attack, and they can utilize normal speech when they have taken a form that allows it. Some also seem to have psychokinetic abilities, affecting the outside world with the power of their minds alone, not requiring the incantations or components a humanoid mage might. Some have evidenced the ability to learn more conventional magic as well.

'Breeding' encephalids seem to carry a large colony of larvae in an internal fluid sac, and instead of siphoning off fluid with the hollow stinger tubes on their pseudopods, they inject them, starting a long and torturous transformation of the victim into one of them, which retains many of the memories of the parent encephalid in addition to the memories of the host, which they then sometimes impersonate. It is unknown whether they require other members of their species to produce larvae or not. To clarify, by the way, the reason they are parasitical rather than symbiotic is that in almost all cases, although the resulting encephalid retains the memories (and when needed, the form and appearance) of the host, the host themselves is destroyed in the transformation, their mind consumed by the encephalid, their needs, ties, and beliefs entirely eradicated.

The body they have suborned, sickeningly, seems to retain the ability to reproduce with members of the original host species, resulting in half-breeds, and in later generations, some with only the tiniest of relation to the encephalid. However, any creature with any trace of encephalid ancestry could, under certain circumstances, undergo the encephalid transformation, partially or fully, and even if they do not transform, they often have greatly enhanced mental powers resembling those of the encephalids themselves. Some seem to be fully normal members of their societies, and possessed of normal moralities. Unfortunately, though, a pull toward selfish and evil behaviors is extremely common, even if they never change. Let us be clear: they have not gone mad. They are fully in possession of their faculties. They simply seem, in those cases, to be completely with out pity, remorse, or any vestige of conscience that a humanoid would recognize, motivated entirely by their own need and sometimes alien urges that we cannot easily understand.

M&M sheet here with the stats for PF2 and DnD5e as well. https://www.patreon.com/posts/77978427