Sineaters

Summary

Sineaters were a consistent character in several myths and legends. Their main abilities as magical creatures were centered on soul magic. They could eat a person's sins, like a magical surgery for a person's psyche. They could cut out the parts that a person didn't want or considered "cancerous", though sometimes that part of the psyche would grow back to similar or even worse proportions than before. A very "careful what you wish for" vibe.
In most stories they are a neutral or evil character for the protagonist to overcome or be tempted by. In more recent horror takes on the characters, the fact that sineaters could blend in with every day folk, have gained popularity. The idea is that you wouldn't know a person was a sineater until they began to feed, their pupils would expand to full iris and the veins would darken beneath the skin (even if the skin was already dark). 
However, there are also tales that sine eaters are the only cure for sun sickness and that any currently living sineaters live deep in the Blistering Dunes.

Historical Basis

"They say if there are any sineaters left in the world, they live where humans can't hunt them, deep in the Blistering Dunes." --Merchant at Deefin Gate

Sineaters used to exist and were healers before the fall of Chog. Because of their magical abilities, they were ostracized and hunted down. Today, they are very rare because they mostly died out before the fall. In fact, their existence is now debated and many think they are completely fictional. 
While sineaters are magical creatures, any one can "evolve" into a sineater, though some are more predisposed to it than others. The individual must be highly empathetic so that they can "feel out" the soul and find the pieces in need of excision or balancing.

Spread

This is a fairly common myth  though there are different takes in different regions. In Piromere, it is the horror of thinking someone is normal only to find out that they are some horrible monster that feeds on another's soul. In the Borderlands and the Wastes, it's the issue of thinking someone else can solve your internal turmoil or problems.