Doppleganger

Doppelgangers, otherwise known by themselves as Shallar, are monstrous humanoids, infamous for their shapeshifting abilities that allow them to mimic almost any humanoid creature.There is also a stronger, more capable subspecies known as greater doppelgangers or mirrorkin.
In their true form, doppelgangers appear as tall, elven, gray-skinned humanoids, whose thin hairless bodies make them appear, to human eyes, sexless. Their heads are bulbous, with formless faces and bulging eyes that are a pale yellow hue and lacked visible pupils. Though even this form was deceptive, as it gave the impression are physically weak, when in fact they are quite strong and agile.
They are lazy but cunning creatures, who kill or dispose of people then assum their place. While not actually evil, doppelgangers are extremely self-centered and liable to look down on their victims. Because of their laziness and selfishness, they rarely ever have any interest in raising their young.
The union of a doppelganger and another creature will produce a child that is either a doppelganger or what is known as a half-doppelganger. However, such children will be unaware of their true nature until they reach adolescence, at which point they will try to seek out others of their kind.
A doppelganger in service of Gornak, shapeshifting out of his human disguise.
Doppelgangers can assume shapes of any humanoid creatures between 4‒8 ft in height. After they choose their victim they duplicate  the victim's form, clothing, and equipment, and attempt to kill the original. The biggest boon for the doppelgangers is the confusion their shapeshifting causes.
These shapeshifters are immune to many mind affecting spells, such as sleep and charm spells. They also possesse a continuous spell-like ability to detect the thoughts of their victims and opponents, which some describe as being a form of ESP. This ability extends out 60 feet (18 meters) and can penetrate most 2‒3 ft (0.61‒0.91 m) barriers. These will often use this ability to learn everything they can about their chosen target.
Though their abilities allow them to physically impersonate other creatures, a normal doppelganger can not duplicate the languages, memories, mannerisms, or personality of their victims. Because of this they sometimes keep the original alive and close by for weeks when their plans were more long-term. While captured a victim's mind would be read daily by a doppelganger in order to learn how to best behave and speak without being detected.