Shapeshifting humanoid aberrations, the nature of which the characters knew very little at first.
One was encountered (and defeated) by the party in the basement dungeon on the first day of the adventure.
In Chapter 5, another was defeated in the Briarstone nurse’s station.
In Chapter 7, a third was slain in the laundry area.
In Chapter 19, a fourth was put down in the chapel.
As the party advanced, they learned more information about the creatures: high natural armor, deadly claw attacks, ability to perfectly mimic other humanoids. Automatically proficient in all weapons, armor and shields, and can use any spell trigger or spell completion item as if the spells were on their spell list. Able to cast detect thoughts at will.
The doppelgangers in Briarstone Asylum, having been “created” under the unusual nightmare circumstances, tended to be mostly (all?) former doctors. Dora suspected that it was their significant egos that refused to allow them to stay concealed before killing someone.
First Reference: Chapter 1
Other Notable References: Chapter 5, Chapter 7, Chapter 10, Chapter 19