They craved attention. They were what you see in the corner of your eye. The audio and visual tricks played in your mind. It wasn't your mind. It was a beg. A beg for attention from a being separated from our reality. An entity essentially looking through a one-way-mirror. Longing to be on our side. The lack of attention drives them mad. They snap. The world ends. They regret it. Was it really worth it? Doctor... I miss them...
A being born of darkness is not visible calmly. A being of black mist is a god of its realm. Of its Mobius. Every entity claims its own realm. One was unlucky enough to get mobius. A place where they're banished from the mortal plane. When an entity is banished, they have no control of the surrounding domain. They can only watch and make themselves known out of full view. That is, of course, unless the rage within the entity is built up to flames. Then the entity manages to break these laws.