Building a better Myconid
I mostly dislike Forgotten Realms, and I especially dislike Myconids. Bipedal? Limited to mushrooms? They hardly do justice to Kingdom Fungi.
This is an alternative implementation of fungal sentience, for use in (preferrably non-Forgotten-Realms) games of D&D.
Superfungi
Superfungi are fungal colonies which have grown so deep and vast that their mycelial network has achieved sentience over its thousand-plus-year life. They vary wildly by species in habitat, temperament, and life cycle. They are particularly rare. There are probably only a few hundred to a thousand such fungal colonies in the entire world, but each one can cover several square miles of surface terrain, and untold depths below.
Superfungi are both a network (sometimes facilitating communication between plants like regular mycorrhizal fungi do) and an individual with a single "voice", ambition, and memory.
Their role in the local ecology is to assist in the exchange of nutrients between plants and recycle the dead. Their general alignment relates to whether or not they see the moral imperative of that role to be enabling all beings to flourish equally (neutral good) or maintaining a smoothly working system (lawful neutral). But superfungi colonies are distinct and unique, even among the same species, so their alignments are just as varied as people's.
When we talk about superfungi's symbiotic and parasitic relationships, remember to distinguish between societal symbiosis (as in humans and dogs) and composite organims (as in lichen).
Superfungi can be killed, but due to their size and scope it's not a matter of physical combat, and would probably be considered an ecological crime to even attempt.
Drones
Many fungi produce fruiting bodies called mushrooms which release spores. When superfungi grow giant ambulatory mushrooms, they are called drones. While they are attached to the mycelium they can serve as sensory organs, but as soon as they detatch they become more or less preprogrammed robots. They have all the parts and logic built into them to perform a specific task (usually just spreading spores), but they are incapable of their own thought, reasoning, or reaction. A drone inherits its alignment from its parent organism.
Like most mushrooms, they grow their entire structure within the mycelium compressed as a "pin", which inflates with water to its final size when activated. A mycelial colony grows drones of different types in advance, and activates them as needed. Once activated, their machinery performs the intended task and winds down like a spring powered clockwork toy, then simply stops.
Because they must be preprogrammed, making a drone for a one-off task is expensive. A drone could be "programmed" to engage a foe in combat, but given the complexity required there would have to be quite a dire need to produce a drone as a fighter.
Drones vary in shape by species: some could be bipedal, or quadrupedal, or light and fluffy and depend entirely on the wind for movement, or they could roll, or they could be entirely alien shapes.
Like mushrooms, drones have a short life of a few days (or less, depending on the weather) before it rots away, so they're seldom seen far from their parent mycelium.
It is a technological art of the superfungi's culture to create more advanced types of drones. Whether or not a superfungi shares new advances in dronecraft with other distant superfungi depends on its temperament.
Myconoids
Animals which host superfungi on their bodies are composite organisms called myconoids (that's ‑oid, not ‑id). The types of relationships are highly varied (again, by species):
Parasitic
When the superfungi parasitizes a plant or animal's body, it either draws nutrients from the host at the expense of the host's health, or it grows reproductive structures on the host and uses the host's mobility to spread its spores.
When the superfungi parasitizes a host's mind, it exerts influence over the host, ranging from subtle suggestions to outright mind control. The spores that infect the host usually only carry the instructions for a single preprogrammed goal, but will be able to use the animal's reasoning, imagination, and instinct to accomplish it. The goal is not always as simple as "spread more spores".
Symbiotic
It's much more common for superfungi to infest a host's body or mind in a mutualistic relationship. Animal hosts naturally offer fungus a source of water and nutrients, as well as the mobility to spread spores far and wide. In exchange, the superfungi may offer nutrients, environmental filtration, medicine, or psychoactive drugs; or if it infects the host's brain, it may offer knowledge, conversation, and the ability to converse with other superfungi through touch.
When a superfungi colonizes its host's mind in a symbiotic relationship, it does not exert any influence over the host, but instead allows it and the host to converse in real time. They don't have to be friendly with each other, but they are generally interested in each other's welbeing.
Rarely, a superfungi merge so completely with the host's mind that they become a new and unique hybrid organism. It's hard to say if this was consentual, or even intentional, since neither the host's nor the superfungi's consciousness are left around to say so. The brand new organism shares its host's and fungi's instincts, but not its personality, aspirations, or soul. You might call it "feral".
Communication
Superfungi, if they are a mycorrhizal species, can talk to plants connected to its network through a language unique to plants and fungi. A being without hyphae or roots can never comprehend it, regardless of any skills or spells that grant language proficiency. Fungi aren't plants, but a creature with the ability to speak with plants may be able to communicate with one using plants as an intermediary, even without realizing it. Superfungi may take offense to being called a plant, as well as the insinuation that the spell Speak With Plants granted them sentience that they didn't already have.
The concepts of "self" and "other" are new and unfamiliar to most superfungi, though they do understand that there are different sources of communication on the mycorrhizal network. When two networks of the same species physically meet, they immediately integrate and have no memory of ever being separate entities.
To communicate with another superfungi over longer distances, they enlist help of myconoids to carry message spores. This doesn't imply that one superfungi knows of the existance of a specific "other" superfungi; it's more like a scaled-up analogy to spreading spores on the wind: they're sending a message, and who's to say if it finds a home. Usually this messager task is done by willing symbiotic myconoids, generally hermits or feral people who have shunned or been shunned by their society.
Superfungi can communicate with animals through symbols, writing, or spores, or more exotic forms such as speech.
Symbols and writing
The superfungi can emboss a mycelial surface with words or pictograms by inflating and deflating parts of it with water. This is a very slow mode of communication.
Spores
Communication spores can be released from mushrooms which bear a message. Any sentient being which inhales them will receive that message effectively telepathically (because the spores "grow" the thought right into the recipient's brain). This is slower than conversational speech, but faster than writing, and there's no way for the mushroom to target a specific recipient for the message. This is not necessarily a hostile infestation attack, but it can be resisted by an unwilling recipient with a Constitution save.
Speech
According to legend, a superfungi colony was once displeased with the expansion of a neighboring animal village without being consulted about it.
One day, a fungal drone walked through the city gates, navigated its way through the town, and entered a meeting hall where village governors were attending to their business. Using a complex set of tubes and bellows, the drone vocalized a scathing condemnation in the animals' native language, then promptly exploded in a cloud of spores.
The speech, the sheer ostentatious expense of crafting the talking done, and the itching fungal infections that the unfortunate governors hosted for several weeks, convinced the village to divert their growth in ways more to the liking of the superfungi colony.
Sample Species Ideas
Species concepts covering all the obligatory archetypes. Mix, match, rename, and reskin to taste.
Witch claw
This fungus is easily identifiable, as it shrouds its environment in bone white mycelium and matching bipedal drones covered in a distinctive sharp scaly texture. They're utterly and immediately lethal when ingested, and when threatened their dones and other reproductive structures release corrosive spores, but besides that they're regarded as quite gregarious—probably because nothing ever tries to eat them.
Witch claw does not infest individual animals with spores, but are known to form symbiotic relationships with animals at the societal level: an animal society will use the mycelium as a cemetery and offer their dead as nourishment, in exchange for safety and a direct line of communication with their mycorrhizal network. Even non-sentient animal packs can form this relationship.
Cannonball Mushroom
Its drones are small, lumpy balls, which are able to roll with the help of some inflatable protuberances which give them momentum and direction. When it nears a victim it overinflates, exploding with spores over a 20ft radius. Victims who are infected suffer a debilitating infestation, though they retain their mental faculties. If they die without being cured, their body becomes the site of a new mycelial colony.
Like most fungi, they are awfully specific about their growing substrates. In this case, they only thrive in the bodies of arcane magic users, and the drones are programmed to respond to their presence specifically.
It's thought that this superfungi's sentience is not a result of centuries of growth, but rather the result of the arcane magic that seeds each organism.
Tarpit
A carnivorous fungus of the surface world that transforms tracts of scrubland into squishy, swampy patches suffused with digestive fluid. Animals who wander into those patches may find themselves struck immobile and unresponsive by a potent hallucinagen, where they will continue to stand until they succumb to thirst, the elements, or digestion.
The only visible traces of this superfungi are often its gaunt and shivering victims found standing alone in the woods. They are not mind controlled like myconoids, but they will resist, to the last, any attempt to move them out of their hallucinated fantasy.
This species is often in symbiotic relationships at the societal level with animals who lure victims into its killing zones in exchange for safe passage and a share of the captured prey.
Cedar ghost
It grows in dead cedar trees, producing mushrooms which mimic the appearance of live foliage. In daylight, it's indistinguishable from a live tree (without a Nature check), but at night the same mushrooms glow a pale green.
Its reproductive strategy is to parasitize victims with mind controlling spores (with a preference for sentient beings), who then infiltrate the hosts' homes to carry out the mycelium's desires, which can even extend to political intrique and other high level concepts. (In game terms, myconoids retain all the abilities of the host, but are effectively mind controlled to have the superfungi's motivations.)
Like infested cedars, its myconoids are undetectable aside their eyes and saliva glowing a pale green in darkness. Its drones are scarcely seen, but it's assumed that they are the ones who pile the dead infested victims' bodies high at the base of dying cedars.
They are not mycorrhizal, and rarely conversational.
Wisp
A benign symbiotic superfungi that grows in wet clay, named for their fruiting bodies, which are lacy clumps mycelium webbing that aimlessly ride the wind. Animals tend to seek out wisps in order to host an infection, which produces a mild psychedelic experience while conical, blue-gray mushrooms sprout from the affected skin, spreading spores wherever the host travels. Both the drug trip and the skin infection clear up in 3 to 4 days with no lasting ill effects.
The wisp mycelium is typically friendly to animals who are able to communicate with it, and delights in collecting and sharing stories from its long memory. Bear in mind that anyone capable of speaking with it is probably high as a kite at the time.
Sage of the Rock
A superfungi which shapes, and is shaped by, the growth of secluded animal societies. A symbiotic relationship to an animal society is a required part of its reproductive cycle.
Almost all members of the animal society choose to become myconoids. They develop patches of spore-producing hymenium on their neck and limbs, which resemble scales. In exchange, sage of the rock filters or destroys almost all toxins and parasites from the myconoid's blood.
When sage of the rock colonizes the mind of a myconoid, the animal gains the ability to sprout hyphae from its limbs and join a mycorrhizal network. Only a select few myconoids ever receive this gift, who usually then take leadership positions (which could be called a "priesthood") in their society.
Societies that host a sage of the rock tend lovingly to its cultivation, but are secretive and hostile to outsiders, knowing that many animals will resort to violence and conquest if they learn of the superfungi and its benefits.
Red Iron Fungus
It lives underground, permeating ore-rich soil especially in and around caves and tunnels. Its reproductive structures are stringy tubes which might be confused for roots dangling from the ceiling of subterranean caverns, which release spores into the air when it senses vibration.
The red iron fungus's culture revolves around arranging its underground environment to suit its aesthetic tastes, and it releases bizarre 9-limbed drones (having 6 2-jointed legs and three arms, all converging at a single node) to achieve this. Since the mycelium is only concerned with the earth that it permeates, the fraction if its decorations that are visible to animals probably seem haphazard, senseless, or maddening. Animals who find themselves hosting a red iron infection may begin to share this particular obsession.
It is nonviolent but territorial: outsiders aren't unwelcome per se, but when they fall afoul of the superfungi's inscrutable aesthetic tastes, its drones will drag them off to become a more pleasing part of its landscape. (In game terms, they grapple and drag people off to be imprisoned, but aren't interested in fighting.)