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The scenario is this: The heroes have figured out there's a doppelganger, and they know who it is, but the only way they can prove it is to make it reveal itself as a doppelganger.

The party can use spells and magic items and anything else they can think of, but directly attacking with weapons is not an option. Using options that will damage but not kill the doppelganger is allowed, as long as it forces the doppelganger into its original form but are not preferred compared to nondamaging options.

Backstory

The doppelganger is impersonating a dwarven king, and the climax of the adventure when the heroes will make the reveal will be taking place during a feast, in the king's throne room. All of the dwarves present (hundreds of them) have no suspicion that the king is anything other than their beloved ruler. That is why attacking the doppelganger isn't an option. If the heroes attack it, they will be immediately attacked by the guards and other guests and quickly overwhelmed. They need something that will work immediately to reveal the doppelganger's true nature.

Party composition

The party consists of 3 10th level characters. A human rogue/assassin, a gnome druid/circle of the land, and a dragonborn bard/college of valor. The druid has a staff of the woodlands, most of the other magic items they have are weapons or armours or mundane enough to not have a use in this situation.

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  • \$\begingroup\$ What spells does the bard know? \$\endgroup\$ – NautArch Feb 27 '18 at 21:34
  • \$\begingroup\$ @NautArch the druid took the forest type, and I don't have the bard's spell list in front of me, but just assume they have any spell you might suggest. \$\endgroup\$ – fiend Feb 27 '18 at 21:53
  • \$\begingroup\$ Bard's are severely restricted in their spell choices, so that's not quite reasonable - but it doesn't entirely affect answers as scrolls/casters for hire might be found. \$\endgroup\$ – NautArch Feb 27 '18 at 21:54
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    \$\begingroup\$ Please don't signal your edits in text. If you have an edit, don't prepend it with EDIT; if you have more information that might be relevant, just put it in the question. We want questions to be perpetually in a "final draft" state and look as though they were that way all along, and we have an edit history that makes it clear what was edited if anything. \$\endgroup\$ – doppelgreener Feb 27 '18 at 22:08
  • \$\begingroup\$ Maybe dopplegangers work different now than I remember, but how long has it been impersonating the king? Surely there's a friend or relative of the king that will remember an event or story that the copy has no way of knowing what happened. It's a well-worn movie/TV trope, "Remember, Tom, when you took Sally to prom and...blah blah blah." "Oh yes, it was quite the event." "Ah ha! The real Tom was in the hospital having his appendix removed, he never went to prom!" \$\endgroup\$ – sirjonsnow Mar 1 '18 at 15:19
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Moonbeam!

The 2nd level Druid spell Moonbeam (PHB, 261) has the following feature:

A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can’t assume a different form until it leaves the spell’s light.

Doppelgangers (MM, 82) are shapechangers and would therefore undergo this transformation upon failing the moonbeam save.

Your druid has the easiest access to this, although the Bard could use a magical secrets to learn it as well.

Caveat Emptor

Given that the party is surrounded by potentially hostile forces, this is still a risky play. Should the Doppel make their save, then there would be no effect except for a lot of angry dwarves. I'd suggest contigency plans to allow for another casting (maybe a Tiny Hut created by the Bard) that the team could safely remain inside while the Druid attempts another moonbeam.

You'd also need to ensure that the Doppel starts it's turn in the moonbeam.

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    \$\begingroup\$ It's worth noting in your post, that the spell has the potential to do damage. This damage can be avoided by cancelling the spell before the end of the turn where they cast it. \$\endgroup\$ – goodguy5 Feb 27 '18 at 20:52
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    \$\begingroup\$ Actually, it seems the only way to invoke the saving throw is to invoke the damage, which would fall under "unable to cast spells that cause damage". \$\endgroup\$ – goodguy5 Feb 27 '18 at 20:53
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    \$\begingroup\$ Note that pinning a TPK on a single roll, even one with disadvantage, is not a good idea. Be prepared to get this spell off at least twice and preferably three times before any dwarves take actions. \$\endgroup\$ – Please stop being evil Feb 28 '18 at 4:16
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    \$\begingroup\$ If I were DMing, I would potentially go so far as to fudge the save to guarantee that the doppelganger fails so that the narrative can't go off the rails. Maybe handwave it by saying that the doppelganger was completely surprised by Moonbeam and didn't have the presence of mind to resist. \$\endgroup\$ – Kevin - Reinstate Monica Feb 28 '18 at 20:38
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    \$\begingroup\$ If you're worried about doppy fleeing, have the party pretend like this is all a cool ritual that they use back at home to honor the "true king of the land" or something. The doppelganger would want to sit perfectly still for it. (Naturally, the bard will expound upon the wonders of the ritual in High Chant.) \$\endgroup\$ – EthanB Mar 1 '18 at 0:10
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Use Glyph of Warding

Let the Bard cast Glyph of Warding with detecting Doppelgangers as part of the trigger condition and using a harmless but obvious spell like Faerie Fire in the Spell Gylph (as suggested by KorvinStarmast).

You can further refine the trigger so the spell activates only under certain circumstances or according to Physical Characteristics (such as height or weight), creature kind (for example, the ward could be set to affect Aberrations or drow), or alignment.

As seen above, the spell can detect specific creature kind, so by RAW it can detect Doppelgangers. Use this to threaten/negotiate with the creature to make it revert to its true form.

What would be better is to get (using suggestion perhaps) a dwarven mage to cast the spell for you, making the result more convincing (added due to NautArch's comment below about convincing onlookers).

Edit: My original answer was to use suggestion, which it is immune to, as pointed out by NautArch.

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  • \$\begingroup\$ I'd suggest bundling in Faerie Fire so that the glyph going off doesn't harm the critter, which seems to be a criterion. \$\endgroup\$ – KorvinStarmast Feb 27 '18 at 21:59
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    \$\begingroup\$ Just have to convince onlookers that the glyph works like that. Great idea! \$\endgroup\$ – NautArch Feb 27 '18 at 22:01
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    \$\begingroup\$ Only problem I see is that only the Bard will know that the trigger is a doppleganger. Every one else will just be told that the trigger is a doppleganger. There will be no proof... \$\endgroup\$ – Shem Feb 28 '18 at 0:43
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    \$\begingroup\$ @Shem As mentioned in the answer, the better way is to get a local dwarven mage to cast the spell for the group, either using suggestion or good persuasion. \$\endgroup\$ – Tenryu Feb 28 '18 at 2:11
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You don't need to not kill the doppelganger

You have an assassin in your party, and your goal is to eliminate an imposter head of state without getting killed by his guards and/or army. You don't need spells for this, you need poison or a sharp knife and expertise on stealth checks.

Killing the doppelganger in this situation is pretty much exactly what the non-evil version of the assassin rogue adventurer archetype is supposed to do. This should be that character's time to shine and, while ideally the rest of the party ought to not be left out, this is the sort of set-up I would pitch if I wanted to give that particular player some extra time in the spotlight and power in party decision-making for a session. Dead doppelgangers revert to their true form, as a bonus.

Note as well that he doesn't have to kill it in one hit, and he certainly doesn't need to fight it in the drinking hall. He just needs to set it up so that he can kill it without the army attacking. Maybe he poisons the 'king''s food/drink with a laxative, then hides in the latrine. Maybe he stays under the 'king''s bed until he goes to sleep then murders him. He might be able to do it in the meeting hall, but it's better to set things up so that the king is alone and the guards are distracted, and then risk it.

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  • \$\begingroup\$ I considered this, but it's very dependent on how the dice fall for the damage. A doppelganger has 52 HP, and that's on the high end of how much damage the rogue's crit sneak attack can do. Very risky, but I guess they might try it. \$\endgroup\$ – fiend Feb 28 '18 at 12:59
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The first level spell Command does what you are looking for, if you've got a spell scroll of it:

You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is Undead, if it doesn't understand your Language, or if your command is directly harmful to it.

Some typical commands and their effects follow. You might issue a command other than one described here. If you do so, the DM determines how the target behaves. If the target can't follow your command, the spell ends.

You will need to devise an appropriate one-word command for this, which you should work out with your gamemaster. 'Revert' might work. 'Polymorph' might be good enough. 'Unpolymorph' is probably your best bet.

If you don't have a spell scroll of it, you'll need an NPC to help cast it; Cult Fanatics can do so, as can lots of not-humanoid NPCs.

Beyond this first level enchantment, you're limited basically to things that could duplicate its effects, illusions to make the doppelganger look like a doppelganger, and making the people you want to convince able to see that it's a doppelganger without it reverting form.

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    \$\begingroup\$ I like this, but I think there's a risk that reverting could be perceived as directly harmful to it. \$\endgroup\$ – NautArch Feb 27 '18 at 21:49
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    \$\begingroup\$ @NautArch There's a reason they specify directly. It is likely to be harmful, but doppelgangers can shift all the time; the harm will be indirect, after others respond to the revelation. Direct harm is something like jumping in lava or sticking a sword through yourself. \$\endgroup\$ – chrylis -on strike- Feb 28 '18 at 7:46
  • \$\begingroup\$ @NautArch It's true that there's always that risk, it without knowing more about the individual doppelgänger personally we can't really know. I think the risk is probably pretty slight; this is only about direct harm so it doesn't matter that the guards will kill them afterwards but it does if they've got some kinda phobia of showing their true form or something like that where it'd be direct emotional harm. We have a question on that about how to stop command as a barbarian, somewhere. \$\endgroup\$ – Please stop being evil Mar 6 '18 at 17:03
  • \$\begingroup\$ Yah, I think that it's probably a safe call. And I absolutely love the Command spell. \$\endgroup\$ – NautArch Mar 6 '18 at 17:33
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Tricky workarounds

If your bard can cast Mass Suggestion, or if you can pay someone to cast it, use that to convince everyone around that the Doppelganger IS a doppelganger and is now exposed. With everyone calling him out, he may return to form as he believes the jig is up.

A possible phrasing could be "You know that Person X is a doppelganger. Expose him by Trial by Moonbeam and protect your village!"

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    \$\begingroup\$ I like the Moonbeam option. Yes I said that the doppelganger couldn't be attacked and damaged, but that was only because I was assuming that killing it would be the only way to make it revert, and the party has no way of doing enough damage currently to kill the doppelganger in one round. But if the druid uses Moonbeam, it will damage but not kill the doppelganger, but it will also force it (subject to a failed saving throw) to revert. \$\endgroup\$ – fiend Feb 27 '18 at 22:06
  • \$\begingroup\$ @fiend if you can update your question to allow damage, I'll undelete my first moonbeam answer. \$\endgroup\$ – NautArch Feb 27 '18 at 22:16
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A simple, nonmagical option is to bring the real king to the feast, whether dead, alive or resurrected. At the very least, a confrontation ensues to establish who is the real king. This works better if the real king is alive (or recently made so), can account for how the switch occurred and conveys knowledge the doppleganger could not. Extra points would be awarded if the presented "king" wasn't the king at all, but rather another impostor created somehow by the group.

Obviously, this answer entails a measure of creative control only a Dungeon Master/Storyteller/Game Director/whatever would have.

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  • \$\begingroup\$ Unfortunately for the party, they don't know where the real king is. I suppose the bard could try to impersonate the king with some illusions or something. Interesting suggestion! \$\endgroup\$ – fiend Feb 28 '18 at 13:01
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Cast sending to contact the true dwarf king in full view of the court. They should immediately notice something's wrong when the doppelganger on the throne doesn't receive it. This works if the king is alive because it went somewhere else. This works even if the true king is dead because the spell will fail or go to another plane, neither of which go to the doppelganger. If the spell fails because they don't know the king and the people of the court would believe they did because the know the doppelganger then again the spell didn't go where it was intended to go.

This only works because the party knows the true king isn't on the throne.

This doesn't work if the members of the court would not believe the players know the king well enough to cast sending to him.

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    \$\begingroup\$ I'm not sure that Sending would work unless they've met the king. It requires familiarity and I don't think knowledge of the King counts (I don't think I could cast Sending to speak to the president of my country.) They also don't have to answer...and the Doppel could fake it with a Deception check that might be at advantage if they're within 60' of you. \$\endgroup\$ – NautArch Feb 28 '18 at 21:12

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