Doppelganger

Doppelgangers are shapeshifting, mind-reading monstrosities who infiltrate humanoid societies. Sometimes they replace people to assume their position and wealth, and sometimes to engage in longer-term plots.

Appearance
In their true form, doppelgangers have milky white eyes, featureless gray skin, and wispy white hair, with a humanoid figure.

Abilities
All of the following is taken from the Monster Manual, 5th Edition, and may not be exactly canon for Critical Role, as no doppelganger has been encountered onstream.
 * Condition Immunities: Cannot be charmed.
 * Shapechanger: Polymorphs into the form of a Small- or Medium-sized humanoid it has seen, otherwise keeping its own equipment and statistics. Reverts to its true form upon death.
 * Read Thoughts: As an action, magically reads the surface thoughts of a creature within 60 ft., as long as there are no sufficiently thick/dense barriers in between. Can continue reading that creature's thoughts as if concentrating on a spell. While reading that creature's thoughts, has advantage on Insight, Persuasion, Deception, and Intimidation checks against the target. (Doppelgangers are proficient at Insight and experts at Deception.)
 * Ambusher: In the first round of combat, has advantage on attack rolls against creatures it surprised.
 * Surprise Attack: Extra damage against creatures it surprised.

History
Vanessa Cyndrial, Kashaw Vesh, and Zahra Hydris killed a doppelganger and brought back its severed head as part of a contract for the Slayer's Take. Kashaw and Zahra were each paid 2,250 gold for their part in the hunt. The creature had been posing as many different people across Vasselheim, changing itself at will.

As of 812 PD, the drow of Ruhn-Shak were so plagued by shapeshifting, mind-altering aberrations as to feed a deep paranoia that was tearing apart their society. Some drow commoners, in despair, offered themselves over to the aberrations and were transformed into doppelgangers.

At around the same time, a doppelganger with delusions of grandeur replaced a member of the Catyurit family that farmed chellerum gourds outside Emon and began poisoning some of the produce as part of a strange plot.

The Darrington Brigade, operating out of Deastok, foiled a doppelganger's plot against an ore baron.

As of early 836 PD, the Archmage of Industry on the Cerberus Assembly, Baroness Jenna Iresor, appears to be human but is actually a doppelganger. She magically tampered with people's memories to help create a false identity, and enjoys the lavish lifestyle she has attained in her new position.

In the Explorer's Guide to Wildemount sourcebook, the low-level adventure hook Kryn Spy involves a doppelganger working for the Kryn Dynasty and perpetrating arsons in the Felderwin Tillage, a major source of food for the Dwendalian Empire.

Notable doppelgangers

 * Baroness Jenna Iresor, Archmage of Industry on the Cerberus Assembly
 * "Foryuna Catyurit", a doppelganger engaging in a poisoning plot in Emon