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.
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.
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.