Zuggtmoy is the queen of fungus. Her inhumanity rivals that of any demon lord, an amorphous mass that only occasionally deigns to present itself as somewhat humanoid. She has no set form, no race who pledges its undying loyalty, no meaningful creatures on the Material Plane because creatures mean nothing to her. She seeks a world cleansed of all animal life, leaving only plants and fungi as part of a single universe-spanning organism.
Her methods are even more insidious than those of other demon lords. She does not intentionally attract worshippers, because anything capable of worship is probably anathema to her. Instead she uses spores to infect, destroy, and control anything and everything she can. The bodies themselves are just a useful tool, spreading her sickness far and wide, increasing her reach until nobody can escape her authority.
Typical Followers: Almost any creature can be an unintentional follower of Zuggtmoy. Anything with a metabolism can be home to spores that control its body and brain: a human, a goblin, an elephant, a beholder, anything.
I’m trying to avoid using any 5E material beyond the core rulebooks and the demon lords themselves, but I have to give a special mention here to the Circle of Spores archetype for the druid. It’s a perfect fit.
Atypical Followers: Intentional followers of Zuggtmoy are much, much rarer. They generally fall into two camps: the people insane enough to think a world without animals is a suitable price for a world without society, and the people stupid enough to think they can maintain their identity when she controls their every movement and thought. Both are valid tragic villain archetypes when done right, but strawman disappointments when done poorly. She probably has fewer than a dozen such followers in any given campaign setting.
A weirder, but possibly more interesting option, is letting her spores infect creatures without metabolism. An intelligent undead may have goals in line with hers and may not interpret the loss of control any differently from being a slave to its own needs. Infested golems wouldn’t be followers per se but they do give her a different sort of minion. Only certain types of elemental seem completely immune—it would take a special kind of spore to take hold in a creature made entirely of fire.
Plots: The nice thing about Zuggtmoy’s spores is their intentional ambiguity. It’s unclear how Zuggtmoy’s spores spread, so you can decide they work in whatever way suits your campaign. If you want combat, her spores spread rapidly and they can turn any creature into an aggressive beast within minutes or seconds, forcing the party to fight their way through. If you want mystery, they spread only at designated times, and the party must find the carrier in a busy city before he infects dozens of others. If you want set pieces, they spread dramatically, filling the area until it’s not clear what’s mundane and what’s an infected creature until the party is already on top of it. If you want tension, they spread quietly, and she maintains just enough control over her thralls for them to seem completely normal until they strike*. The application to horror is obvious, but even a light-heartened comedy campaign can use Zuggtmoy by saying her control manifests as a simple mushroom on a creature’s head a la Earthbound. And nothing says you can’t mix and match these as your campaign warrants.
From the Vault: Zuggtmoy isn’t the only creature that controls others via spores. The 3E Field Folio had the yellow musk creeper, a plant that released pollen to enthrall creatures, draw them close, and steal their mind. Those who survived the process became yellow musk creatures, slaves to the creeper, and its primary means of defense and reproduction. To mimic this, use an awakened tree and give it a dryad’s fey charm attack. Any creature who spends a full 24 hours charmed gains the spore servant template from the myconid entry, except it also becomes eternally loyal to the yellow musk creeper. For a more powerful monster, add an aboleth’s enslave attack to a treant. In both cases, remove the vulnerability to fire damage and add a troll’s regeneration; a yellow musk creeper is still weak against fire (and acid), though it doesn’t manifest the same way. This should increase its challenge rating by one, but you may adjust this depending on your party.
* — I like Skrulls, so before I settled on doing March of Madness I thought about recognizing Captain Marvel by doing a post about using them in games. Here’s your compromise.