In the game, dopplegangers are a type of fungus that read the minds of your companions to construct the perfect image of you. They want to be you.
They want to be you so bad.
|From Dungeon Meshi|
Encounters with the doppleganger fungoid start out the same way. Adventurers tromp through a patch of them when they're in ther larval state. At this point, they look like some mildew on the wall, just barely probing their dark world with their flagella. Then, they get a sniff of the adventurer. A little bit of psychic residue stains them. They push upwards. They grow. They follow.
Doppleganger fungoids use psychic images to choose their appearance, behavior, and demeanor. They, essentially, form themselves into clones of the first people they encounter as mushroom children.
Adventurers often encounter doppleganger fungoids while they camp. If there's ever darkness, ever a moment of sleep, the fungoids slip in and bed down. They want to belong.
The fungoids will try and tag along with adventurers for a while. At first, they'll even be helpful. They continue to refine their appearance and behavior on the original. They love to be as close to perfect as possible.
But they make mistakes. This infuriates them. They develop body dysmorphia. They develop self-image disorders. They get confused, genuinely confused, about who is the original and who is the clone. They begin to self-harm. They begin to lash out.
What does it look like?
Well that's the question, isn't it? What does it look like? It looks like you. A little taller than you. Its left boob is a bit lopsided. Your eyes are hazel but its eyes are brown. But other than that, it looks like you.
That is, until you stab the imposter. And mash it. And burn it. Then it reveals its true form: moldy, tubular stalks of fungus. A lopsided mushroom head. A face like a carven jack o' lantern. Hallucinogenic spores spilling out of it.
They are a painting of you in fungus.
The doppleganger fungus derives its image of self not from the original's self-perception, but from the perception of their companions. To run encounters with these creatures, ask the players to each text you a description of themselves and a description of each of their guild mates. A player's description of themselves becomes canon for what the original looks like. A player's description of their companion becomes a description of one bloom of doppleganger fungus.
For example, if you are running the game with four players, each player would text you a description of their own character and a description of each of their three companions. Then, when you wish to reveal the fungoids, you would reveal twelve new creatures—each described as one of the originals, but seen through the eyes of another player.
Likes: Dampness, rain, tearing things up (obsessively shreds paper, sticks, etc.)
Hates: Fire, mirrors, probing questions about childhood
Special: Make it clear to the players that during an encounter, a "loyal" fungoid will follow the commands of the original like a henchmen. Do not tell the players that the dopplegangers are imperfect. Roll a dice. If the result is odd, the doppleganger misinterprets the command or makes a mistake. This usually ends up with them attacking an adventurer by accident.
Attacks: Pretending to attack as base creature, 1d6
AC: Same as base creature
Save As: 4th level Fighter
# Encountered: The dopplegangers appear in a crowd equaling n * (n - 1), where n = the number of adventurers. See above.
Minor Illusion: A doppleganger can replicate through illusion any item that the original could believably have. A doppleganger can replicate through illusion any class ability that the original has demonstrated previously.
Perfect Imitation: If they have not yet acted during the turn, dopplegangers may elect to move simultaneously to their "original." When the original takes an action, they also take that exact action. If this happens, the original and the dopplegangers have become mirror images of each other. If they are in the same zone, it is impossible for an outside observer to tell who the original is. Essentially, the clones act like mirror images.
Fungal Regeneration: A doppleganger may spend an entire action thinking about who they really are (you). They heal 1d12 HP.