Most likely Error! Cocoa. She appears out of f**king no where, torturing people. That is Maybe Reverse Cocoa. She literally comes out of peoples mouths.
The Mistress of shadows, she seems to be made of literal shadows and can push emotions into people, making them feel ultimate fear, pain, or hopelessness.
I'd say for my oc's It would be Charniva:
1. Will kill you for no reason or due to her messed up sense of justice
2. Mental traumatization is her solution to everything (Think Yami season zero)
3. Three eyes that rarely blink and always keep eye contact
4: DeMoN
5: can use other people's nightmares and emotions to scare you.
6: Well if you have roleplayed with Charniva, list a reason.