I call it a shadow dweller
It is by far the most dangerous entity but it can be avoided easily because it wants to just be left alone these entities hide in shadows to avoid contact with any other beings and the less entities in a level the more likely you will see one
Remember: don't stare at them don't approach them and they want you to leave them alone more than you want them to leave you alone
They get their own level:
Class=deadzone (enviormental, psionic and shadow dweller hazards)
It's called level darkness or level ?
Here is the entity in it's most common tangible form: