how does one ID a doppleganger in disguise?

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Doppelgangers are a load of fun, and I've used them in the past to mess with the party...the party picked up on it and soon learned to have code words to ID themselves from the doppelgangers...

I'm wishing to use them again in my campaign but this time, have them take over key positions in the government. Part of their quest will be to 'weed them out'...Which got me to thinking, what are all the ways that they can 'weed out' a doppleganger...which naturally lead me to Blade Runner style skill challenge...However, the PCs have had NO interactions of any kind with any of these leaders prior to this... which leads to the conundrum...if you ask someone some personal information that only they and the intended target would know, you have to be able to trust that someone...in cases of extreme paranoia, this is problematic...

So, other than asking them a series of questions that only the 'real' leaders would know, what other methods are there?

Can you dispell magic to undo their guise?
Can you prick them for blood (via Start Trek TNG changlings)? 
Would a piece of plucked hair resort back to it's original coloring?
Would they have certain tics that would give them away (via Harry Potter and Mad Eye)?
Is it possible that the target is left handed but the doppleganger is using their right?

I'm thinking of making it a pseudo skill/RP challenge and would like to have as many ideas as possible...

thoughts?
 
I've been thinking about this, especially recently as I'm considering using mimics in an upcoming game.

Can you dispell magic to undo their guise?

Not really, unless you want it to work that way.

Can you prick them for blood (via Start Trek TNG changlings)?

Which probably got the idea from John Carpenter's The Thing, which got the idea from John Campbell's "Who Goes There?"

This doesn't have to work, if you don't want it to. They can certainly color their blood correctly, and give it the right smells and consistency, and there are no blood or genetic tests in D&D, so this could be tough. Unlike a changeling or The Thing, doppelganger blood isn't a separate creature. Mimics now....

Would a piece of plucked hair resort back to it's original coloring?

This also doesn't have to work. I don't think dead doppelgangers change back, so there's no reason plucked hair (which is already dead anyway) should revert.

But this, and the blood idea, CAN work if you want them to. I bet the players with come up with them, in which case you have a strong incentive for making it work that way.

Would they have certain tics that would give them away (via Harry Potter and Mad Eye)?

They could, though I wouldn't expect them to. That would be the first thing a doppelganger would learn to prevent.

Is it possible that the target is left handed but the doppleganger is using their right?

This also seems like an easy one for a doppleganger to get correct.

I think you're on the right track, though. You could give your NPCs subtle traits that a doppelganger might not know about. An allergy or phobia would be a good one. These could even be magically or hypnotically induced in subjects ahead of time (when you know they're themselves) and triggered later - just don't let the doppelganger find out what they are. Or, it could just be that the trait occurs naturally in the NPCs and players who are paying attention will know that Lord McCleod would never pet a cat as he's deathly afraid of them!

(Think of the Klingon agent in The Trouble with Tribbles. They have sensors, but didn't think to use them until the tribbles gave the agent away.)

I used a doppelganger in one game, who had impersonated the mayor of a small town. The players had been told that the mayor had lost his leg, but when they met him he had two legs. The game petered out and they never asked him about it, but it was going to be revealed that the man had had a warforged limb grafted on - or so that would have been the doppelganger's story.

Certain races have traits that they can use to prove they're not doppelgangers. Eladrin can teleport, dwarves can imbibe poisons, a tiefling could stick its hand in fire. Halflings and other small races are safe because doppelgangers can't change size (though there are probably young ones employed for exactly this task). So, make some of your NPCs into other races - and then have someone find a ring of teleportation that might have been used by the "eladrin."

Nothing's coming up right now, but I bet you could find some superstitious defenses used in the real world against doppelgangers, from times when people actually believed in them. You could simply state that those (or something along those lines) are effective. Or make up your own. Maybe a doppelganger can't cross a line of pure salt, or can't speak while touching iron, or can be seen for what it is in a specially prepared mirror.

If I have to ask the GM for it, then I don't want it.

Call upon an expert - a devil who can spot doppelgangers a mile away... but he has something he wants in return for his help exposing the shapeshifters who are otherwise alright in his book. Something sinister no doubt.

Find and use a magical gem that forces shapeshifters back into their natural forms when its magic is activated. That'll make for an interesting dinner party. But first you gotta find the gem and the doppelgangers want it too for obvious reasons.

For any decision or adjudication, ask yourself, "Is this going to be fun for everyone?" and "Is this going to lead to the creation of an exciting, memorable story?"

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I think adventures vault has a weapon that forces shapeshifters back to true form.
A "simple sparing match" could reveal a changelling posing as someone that could not decline a "friendly challenge"
Also what "class" would the changelling be compared to the person they are impersonating?
A soldier or guardsman would have combat options most infiltrators or assassins could never copy. 
The sea looks at the stabillity of the mountian and sighs. The mountian watches the freedom of the sea and cries.
You should let one of the players play a Doppleganger...

As to detecting them...We always found it was the first guy in  the village who looked your PC in the eye and waved hello. Rule of thumb - if the Peasant is happy - kill him.
The Citadel Megadungeon: http://yellowdingosappendix.blogspot.com.au/2012/08/the-citadel-mega-dungeon-now-with-room.html
I found this a while back, which has great ideas about doppelgangers: The Complete Guide to Doppelgangers (readable PDF).

Basically if your players can detect doppelgangers it's because they want to be detected, usually to mask their activities in other areas. The whole left hander becoming a right hander is just the sort of obvious 'mistake' that would draw attention, while the real plot festers somewhere else.

Also, as doppelgangers have telepathic abilities, the whole ID thing would be easy to fake, but that doesn't mean a doppelganger might not pretend not to know the ID to let the party continue to under-estimate the threat.

The advantage of 'obvious' doppelgangers is that if the players take it at face value, you can still have an interesting campaign, but if they begin to wonder "why are these doppelgangers so obvious?" and you throw in some less obvious clues or rumours, they might become paranoid enough to create their own conspiracy, which you can mess with as you choose or let the players be right and feel clever about themselves. Doppelgangers build their own campaign :D
I find it funny that I've never ran a doppleganger as a true villain in any of my games. They've always been more closer to heroes that skirt the law to achieve justice. Undead don't count cause they smell XD

The best way to work Dopplegangers in is not not make them dime a dozen scrubs that the party can just mollywhomp over and over again. A cabal of Dopplegangers should be a terrifying enemy, able to take on the appearance of anyone, and with enough skill and magic at their disposal to be able to learn all they need to about their targets to make it work.

That being said, since they should be major NPCs, they need major quirks to go with it. A smell, a habit that might not be so noticeable, a limp from injury or other trauma, any of these things can be as slight as possible, but still be a dead give away as to a Doppleganger. The key is to make the discovery important. Several run in's with a Doppleganger that tends to wear an excessive amount of perfume would eventually give itself away to the PCs when they come across it in disguise. Someone with a good passive perception might take notice of the limper, so on and so forth.

Disinformation is also key. One chapter of the manga "Jojo's Bizarre Adventure" had the heroes on a ship where the captain had been killed by the enemy and impersonated. One of the heroes made claim that he could tell he was an imposter because cigarette smoke made the veins of people with their powers stand out on their noses. All the heroes and the impostor panicked, but everyone else, who had no idea what sort of powers they had, looked on puzzled. Even though this claim was not true, because the fake panicked just the same as the rest of the heroes, he was outed as one of their kind. This could work towards not only the favor of the PCs, but the Dopplegangers as well. Spreading false info that one or the other may believe is true, and use the result to their advantage.

Dispel Magic doesn't have any base usefulness towards this, since the Dopplegangers are not changing shape through magical means, but it might a hinder a magical alteration on clothing should the Doppleganger be unable to procure a real set of clothes to match.

All in all, what works and what doesn't is up to you, but don't rule out the possibilities of other options working as well. Things are not always as they seem with Dopplegangers, and that's half the fun.

Good luck, and Happy Gaming
One the PCs try to impersonate a dopple ganger that is impersonating a hero, and convinces them to confide his secrets with him.

The dopple gangers are a kind that can only maintain the disguise for set periods of time, and can be found out of the PCs are sneaky enough to catch them when they change back. 

The PCs find the dopple ganger prison where they're keeping the real people they're impersonation.  Or better yet they're more dopple gangers pretending to be the real people tied up, and get you to arrest the real people.

There is a McCarthy style witch hunt that is secretly being run by the dopple gangers to root out real people.


Sorry if not all these suggestions match up with what you were asking for.
One the PCs try to impersonate a dopple ganger that is impersonating a hero, and convinces them to confide his secrets with him.

The dopple gangers are a kind that can only maintain the disguise for set periods of time, and can be found out of the PCs are sneaky enough to catch them when they change back. 

The PCs find the dopple ganger prison where they're keeping the real people they're impersonation.  Or better yet they're more dopple gangers pretending to be the real people tied up, and get you to arrest the real people.

There is a McCarthy style witch hunt that is secretly being run by the dopple gangers to root out real people.


Sorry if not all these suggestions match up with what you were asking for.

In any case, I like them. The first one reminds me of that Star Trek episode we've mentioned before, with the bugs in peoples' necks.

If I have to ask the GM for it, then I don't want it.

@ Centauri: I seem to recall a post you did a while back involving security that was just awesome. I've been looking for it to link because I think it could be relevant here, but can't find it. It was something along the lines of different levels of security defeating various attempts to infiltrate or the like. It broke it down nicely and in a way I could see useful in a game like this.

For any decision or adjudication, ask yourself, "Is this going to be fun for everyone?" and "Is this going to lead to the creation of an exciting, memorable story?"

DMs: Dungeon Master 101  |  Session Zero  |  How to Adjudicate Actions  |  No Myth Roleplaying  |  5e Monster Index & Encounter Calculator
Players: Players 101  |  11 Ways to Be a Better Roleplayer  |  You Are Not Your Character  |  Pre-Gen D&D 5e PCs

Content I Created: Adventure Scenarios  |  Actual Play Reports  |  Tools  |  Game Transcripts

Follow me on Twitter: @is3rith

Great insight and ideas. Thanks...

I have 3 basic factions, dwarf, elf and human. Since the rest of the players are as clueless as I was/am about them, it'll probably end up on me to drop 'clues' as to how they can detect and then hopefully they can run with it (they are savy).

Not 100% sure how it'll go, but I think the hints that might be dropped would be along the lines of:
1) For the dwarf, I love the idea of the poison. That'll be a great way to out that one.
2) For the Elf, figure out a way to incorporate wild step...or perhaps use the fact that elves "As a young elf, you would have heard and memorized these stories of great elf heroes, their legendary deeds, and the struggles of your people"...tie something in where they mistell the story or have the imposter tell it incorrectly...
3) For the human, we have a druid in our group...perhaps they'll find out he has a phobia and have the druid take that shape to see the reaction.

There is a bit of a time constraint the group will be facing so I suppose a last resort might be to have them find the prision and/or gem that exposes them.

Regardless, thanks again, got some good juices flowing. 
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