Psychology
Humans are very contemptuous of orcs. They are brutes, never shying away from any cruelty. And they are warmongers, always divided against each other, always plunging headlong into another meaningless war of succession.
This is very similar to how myconids view humans.
The myconids are the perfect pacifists. They do not hunt--no throats are slit for their dinners. Nor do they farm, with all of the conflict and exploitation that agriculture entails. They are detrivores. They eat the dirt and flotsam of the world. They eat the dead, and cherish the living.
They are not a hive mind, but they are strong empaths, each and every one of them. Thoughts and memories spill over from one myconid to another, like mead spilling between cups during a hearty toast. Because of this, myconids are much less individuated than humans. Myconids can't tell which of their childhood memories are their own, and they think it is strange that anyone would care. Myconids know how foolish it is to fight over a who wears a crown. Since they blend together, all myconids wear the crown.
"Identity" the myconid would explain, "is a concept that humans invented in order to punish criminals. Myconids don't have criminals."
For the most part, this is true. The same mental spillover that occurs between myconids ensures that there are no outliers among the myconids. A myconid murderer would be quickly discovered--their guilt as loud as a klaxon in the ears of the other myconids. A depressed myconid would soon be equilibrated, with their depression diffused across the entire colony, like a drop of paint in a cup of water.
Cultural values and believes propagate in the same way.
Myconids are much less afraid of death than humans are. It is still unpleasant to die, but when a myconid finally passes, so much of their memories and personality is already enshrined in their friends that it hardly seems like they are gone. In the minds of the myconids, it is so easy to imagine a dead friend--their exact reactions and words--that the tragedy is lessened.
In a very real way, a myconid exists in the minds of their friends. You cannot kill them in a way that matters.
Culture
Myconids that travel away from their colony for a long time tend to become more neurodivergent. These myco-nomads are not distrusted, but they are exhaustively questioned whenever they meet up with a colony of myconids. Fresh ideas are scarce among the myconids.
Like other cultures, however, myconids still have a need for secrets, and for independent thought. A myconid can easily accomplish this by cutting off their head. (A myconid's "head" is merely the fruiting body, used for procreation and telepathy. Their true brain is in their abdomen.)
A myconid that has been decapitated will grow to become a house. Important decisions in the colony are usually made by the Council of Houses. Since speaking is difficult for Houses, they typically only voice their opinions by groaning out their disapproval.
In Centerra, myconids are among the devout (like most monsters). They worship Zulin and obey the Authority's precepts. They believe that they are a singular creature, and will all enter heaven or hell together. That's another part of their communal nature, since myconids believe that they are responsible for each other's sins, they usually see each other as another part of their own body. A strange neighbor is just a spasmodic limb.
The religious leader of each colony is the Cathedral. The leader of the military is the Armory. The leader of scientific pursuit is the Academy. The master of hallucinations is the Apothecary, who also makes most of the foreign policy decisions for the colony.
Biology
Myconids constantly shed spores. These spores sprout wherever they can, and become ordinary mushrooms. These mushrooms will never mature into a myconid, and nearly all of them are highly toxic.
These mushrooms are also the primary food source for myconids. A myconid will wander the world, collecting their sprouted mushrooms (inedible to anyone except themselves) and bring them back to the nearest Silo for redistibution. While wandering, they are of course spreading more spores.
In order to turn a mushroom into a myconid, it must dosed with massive amounts of LSD, graciously provided by the Apothecary. Not every mushroom that is thus dosed will go on to become a myconid. Most of them merely grow brains, and spend their remaining days tripping through alien dimensions, learning impossible truths, before finally dying a mushroom's death.
The other mushrooms--the ones with grounded, plodding minds--pass through this veil of hallucigens and go on to become myconids. They grow eyes, arms, and legs before popping out of the ground and following the nearest adult.
Can any type of mushroom be turned into a myconid? Possibly. Some mushroom species can be turned into myconids quite simply. Others with difficulty. Some seem to be impossible. It's an issue of great debate.
There is a long-running joke among myconids, that if they could just find the right hallucinogens and dosing schedule, they could enlighten humans into something better than a race of barbarous fuckups.
Myconids only engage in sex during the direst of emergencies. Sex is something that you do only after your colony has failed at something momentous, and a fresh start is needed. The Apothecary usually gives the order to begin growing phalluses. A few weeks later, when everyone has finished growing genitals, the shame orgy occurs.
Human sexual habits are understood to occur under different conditions, but it is difficult for them to shake the association.
Combat
Myconids appear in groups of 1d8. Each myconid is Level 1d4, and their body sizes vary consderably, from child-sized to ogre-sized. At each level, they gain a magic die and access to new spells.
Lv 1 - telepathy, charm
Lv 2 - illusion
Lv 3 - invisibility
Lv 4 - sleep
They never, ever use lethal force.
Fellow pacifists can be reasoned with. Parley is possible with other civilized creatures.
But people who use lethal force against myconids will be treated like any other wild animal. They will be hunted down and neutralized. Myconid territory must be peaceful territory.
Their weapons are sleep, illusion, invisibility, and charm. Their first resort is often charm. They'll cast it as soon as they see you. This isn't a hostile act in their mind. If you complain, they may give you a scroll of charm so that you can cast it on them.
In times of conflict, illusion and invisibility are used constantly until the threat is nullified, either by tricking it into a padded pit or distant quadrant. The enemies of the myconids will never know what their caves look like, since they will never see them without the veil of illusion. Sleep is a last resort.
If an enemy cannot be scared off or reasoned with, they will be brought to the Armory. The Armory will administer the Dose, and place the wild beasts in the Garden of Earthly Delights. At this point, the beast will exist in absolute ecstasy for the rest of their natural life (which is not likely to be very long). Although the myconids will supply the ecstastic beasts with water and warm blankets, no other care is taken, and the beasts are allowed to expire naturally.
Afterwards, the corpses will be mulched in the community garden.
Uncommon Spells and the Magnificent Travelling House
Although he is rarely spoken of, the myconids do have a queen, who was annointed after her return from the Holy Mountain. She is known as the Grand Mycina, but most of the myconid will only speak of her enigmatically as The Magnificent Travelling House. She exists as several tons of fast-travelling mycelium, existing exclusively underground. The minds of all dead myconids reside in her, in some form.
The Magnificent Travelling House usually travels to the place where she is most needed, where she appears as extra rooms where there were none before, or as a fungus-covered wall that vaguely resembles a face. She sometimes teaches her people rare spells.
Triumphant Rot
R: 50' T: creature D: permanent
If the target fails its save, it slumps over in ecstasy while mushrooms grow rapidly from its body. For the rest of its life, it will flop next to water sources and sigh contentedly until it does of malnutrition. If a myconid gives it a command, it will sluggishly comply.
Inherit
R: touch T: corpse D: 0
Creates a new myconid from a corpse. The new myconid has a portion of the corpse's memories, but is otherwise a normal myconid. The portion of the memories that are inherited depends on the degree of success.
Degrees of Success:
Highest MD is 1-3 = 33% of memories inherited.
Highest MD is 4-5 = 67% of memories inherited.
Highest MD is 6 = 100% of memories inherited.
Dream Quarantine
R: 1 mile T: all creatures D: permanent
The all creatures in the area are trapped in a dream-realm until the caster chooses to release them. (One save is made for the entire group, using the highest save among them.) This effect lasts until the caster chooses to end it. New creatures entering the area are not subject to this effect. A colony of myconids will enter or leave the dream quarantine as a single group.
This spell effectively allows a myconid colony to trap any number of enemy creatures in a mind-dimension. Myconids are not very powerful in a dream-realm (since they are not very creative), but they cannot be killed in a way that matters. If the PCs are trapped in this way, they may butcher any number of myconids before they realize that the myconids are not staying dead, and the features of reality are plastic.
Since myconids take weeks to dessicate, and humans will die from dehydration after a few days, a prolonged stalemate is usually to the myconid's advantage.
It's a bit risky, however, since there's always the risk that the entire colony will be eaten by badgers while their minds are in the dream quarantine. (The first sign of this is usually a myconid vanishing from the dream.) They may choose to end the dream if they can parley some sort of truce with the PCs. This may involve a
binding oath, perhaps involving rings of civilization (below).
Magic Items
Scroll of Summon Ooba
Ooba is a godling that serves the myconids. She appears as a giant toad, and can be bargained with to eat obstacles. She will never willingly harm a living thing, but can eat nearly anything organic, as well as most stones.
Scroll of Summon Brimbool
Brimbool is an ice demon who serves the myconids. He has many abilities, but is limited to making ice walls and describing all the tortures he would like to inflict. He's gotten quite good at making ice walls over the centuries, however.
Scroll of Mass Diminuation
Up to 10 target objects must save or be reduced to 1/12th of their original size. Lasts 1 hour.
Ring of Civilization (Cursed)
Wearer takes emotional (non-lethal) damage equal to all damage inflicted. Cannot be removed.
Mushroom of Enlarge
Effects gained by eating.
Mushroom of Reduce
Effects gained by eating.
Mushroom Hammer
Large weapon. Deals non-lethal damage. It feels good to be hit by the mushroom hammer. Sentient creatures who are hit by the mushroom hammer must save or become unable to defend against the mushroom hammer in the future (since they want to be hit by it).
Luroc's Finger
Looks like a key. The handle is black iron, the stem is white "ivory" that shifts through all possible permutations. If you loudly proclaim where you intend to go and insert the finger into a flat wall, it will create an extradimensional microdungeon that leads to your destination. Usable once.
Roll a d26 and look up the corresponding letter. That is what the dungeon is shaped like. The rooms are randomly generated pieces of the
Long Halls of Luroc--just use random rooms from random dungeons that you have laying around your house. At the end of the microdungeon is a door that leads to your destination.
Using Myconids
Use them like any other race! Which is to say, as another entrenched power center with their own goals, fears, and quirks. Despite their aggressive pacifism, they are not good guys (but neither are they strictly bad guys). They can be colonizers and manipulators like anyone else. They are not above proxy wars (although they will feel very bad about causing suffering, and will probably provide euphorics to the civilians displaced by the war).
Honestly, look at your game map and see if you can't replace of the human towns with a myconid settlement. They trade, give quests, and die like any other race.
PSA: Spores
You'll notice that these myconids are merely spellcasters--they don't do anything magical with their spores. This makes sense, because spores are not just a fungo-buzzword. Spores are the reproductive units of a fungus. Why do so many people have myconids doing magic with their spores?
I know this is more anthropo-chauvinism, where we humans look at a mushroom and ask "but what does it do?" and the only thing we can come up with is "well, it sits there and releases spores". It makes as much sense as flipping to the stat block for humans and seeing their abilities listed as "Pacifism Sperm", "Communion Sperm", "Hallucination Sperm".
I know there are some dudes who don't do much besides release their gametes, but it's a disservice to characterize the entire myconid species this way.
/rant