Doppelganger

Doppelgangers were monstrous humanoids, infamous for their shapeshifting abilities that allowed them to mimic almost any humanoid creature.

"“Shapeshifters,” the Khelbens replied in unison... “Somehow they disposed of Lady Eidola's attendants and took their places. When I found them out, I led them back into the crying room for questioning. One of them attacked. They rushed for the door, taking my form to confuse pursuit.” “If I am a shapeshifter,” said the fifth, “why did I slay two of my comrades with a crushing hand?” The ninth shook his head. “He slew only those two, and in front of you so that you would believe him. I killed the rest”"

Description
In their true form, doppelgangers appeared as tall, elven, gray-skinned humanoids, whose thin bodies made them appear, to human eyes, sexless. Even this form was deceptive, suggesting they were physically weak, when in fact they were quite strong and agile. The normal appearance of a doppelganger was that of a gray-skinned humanoid with a formless face and pale white bulging eyes with no visible pupils. They had no hair in their normal form.

Personality
They were lazy but cunning creatures, who killed or disposed of people then assumed their place. While not actually evil, doppelgangers were extremely self-centered and liable to look down on their victims.

If food and treasure were scarce, they preferred working with the magically inclined; mainly being used as assassins or in elaborate plots to usurp power.

Doppelgangers were feared for their ability to assume the form of any humanoid creature they encounter. Doppelgangers were used as spies and assassins by many beings all over the world.

Many doppelgangers lived a stolen life. When they encountered a person whose appearance or station they desired for themselves, the doppelganger used their mental powers to learn everything they could about the chosen target. Once they had all the knowledge they needed, they quietly eliminated their target and assumed his or her form, taking their place in life.

When a doppelganger was impersonating someone they sometimes kept the original alive and close for weeks, using their ability to read the victim's mind daily to learn how to behave and speak without being detected.

Combat
Doppelgangers could assume shapes of any humanoid creatures between 4 and 8 ft. in height. After they chose their victim they duplicated the victim's form, clothing, and equipment, and attempted to kill the original. The biggest boon for the doppelgangers was the confusion their shapeshifting caused. Oftentimes doppelgangers worked in groups to ensure their infiltrations' and attacks' success. Doppelgangers were immune to sleep and charm spells.

The shapeshifters had a continuous spell-like ability allowing them to detect thoughts of their victims and opponents.

Society
Doppelgangers called their species the Shallar. Doppelganger families tended not to be close; usually, a male and female meet, engaged in casual sex, then leave one another, with the female being forced to single-handedly rear any resulting newborns—who left their mother upon reaching maturity. Some, however, formed tightly knit family clusters, while others mated with humanoids, taking malicious delight in using their spouse as a cover for their inhumanity. Children born of such unions had a chance of being a half-doppelganger.

History
They were believed to be an artificial race, created by the Creator Race known as the batrachi to serve as spies and assassins.

Notable doppelgangers

 * Bonnie, the leader of a doppelganger gang who worked as a barmaid in the Yawning Portal in the late 15 century DR.
 * Cheg
 * Hlaavin: the half-illithid leader of an autonomous druuth in Ch'Chitl.

Appearances

 * Novels:
 * Passage to Dawn
 * The Abduction
 * Adventures:
 * Lost Mine of Phandelver
 * Waterdeep: Dragon Heist
 * Video Games:
 * Baldur's Gate