Doppelgangers

Physical Description
Doppelgangers are a grey-gelatin in their base form. They have blue veins running around their body, and small bones and tendons twisting about. They stand tall with long, dark black claws on their hands. They have jagged, sharp teeth, and usually a couple stick out from behind the lips. They have dark black eyes and no hair, not even eyelashes or eyebrows. They're unreadable when being spoken to.

Capabilities
Doppelgangers can memorize and adapt the forms of humanoids or creatures they stalk and see for an consecutive time of three or more hours. Depending on how long they spend, the better the disguise. They like to take the forms of humans, sneak into their roles and slowly slaughter everyone related to them. Often they kill the true person they adapted the form of, to tie up loose ends.

Doppelgangers know Arcane Magic in most cases, and become extremely effective at murder and trickery. They can use runes to better their disguise and deepen or lighten their voice. They could also open up suddenly with an Arcane spell, etc.

Origins
Doppelgangers are creatures shrouded in mystery. There's many origin stories. Here's a couple of the best and most accepted ones.

Richtofen's Theory: Doppelgangers are lab-expirments gone wrong. Their gelatin-form suggests maybe it was some sort of food or ooze factory. In one of the only bodily examinations of a Doppelganger, it was seen that they have organs similar to that of an ooze; barely any. They don't eat or sleep, and they only have to drink a small amount of water every week.

The Maiden and the Cube: This theory suggests that Doppelgangers are the result of an interspecies love-affair between a gelatinous cube and a human lady. These creatures are a mix between jelly and humans, and that's most of the backing for this theory. It's very up in the air about it's proof, but it's one of the most accepted stories about the creation of the Doppelgangers.