[RPG] way to force a doppelganger to revert to its true form without killing it

dnd-5emagicmonsters

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.

Best Answer

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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