Ancients in New York

A boy gets off a train, and feels an urge to focus on a thought in his head. He discovers that concentrating on that thought seems to move him into a whole new world. He can do it freely at will and has some fun flipping between his own reality and this new one that seems to exist. Suddenly he is made aware of a presence following him, a monster in the new reality is pursuing him. While invisible in his original reality, the new reality he can see makes it clear that he is being chased. No amount of focusing can shake this creature approaching him, and switching realities doesnt seem to slow t...