Doppelgangers

For my fantasy heartbreaker, His Majesty the Worm, I recently ran the party through a doppelganger encounter. It was fun! We had about three sessions of doppelganger nonsense; one where the players freaked out, and then two more when they weighed the pros and cons of just letting them hang out. I was impressed with how the players handled the situation (TONS of creative solutions) and how the general premise worked at the table. I found that this monster encouraged puzzle-esque OSR play.

In the game, doppelgangers 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

Doppelganger Fungoid

Encounters with the doppelganger 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.

Doppelganger 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 doppelganger 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 doppelganger 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 doppelganger 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 doppelgangers are imperfect. Roll a dice. If the result is odd, the doppelganger misinterprets the command or makes a mistake. This usually ends up with them attacking an adventurer by accident.

HD: 4
Attacks: Pretending to attack as base creature, 1d6
AC: Same as base creature
Save As: 4th level Fighter
Morale: 9
# Encountered: The doppelgangers appear in a crowd equaling n * (n - 1), where n = the number of adventurers. See above.

Abilities:

Minor Illusion: A doppelganger can replicate through illusion any item that the original could believably have. A doppelganger can replicate through illusion any class ability that the original has demonstrated previously. 

Imperfect Imitation: If they have not yet acted during the turn, doppelganger 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 doppelganger 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 doppelganger may spend an entire action thinking about who they really are (you). They heal 1d12 HP. 

From Dungeon Meshi