The Black Thing Symbol Analysis

It was a shadow, nothing but a shadow. It was not even as tangible as a cloud. Was it cast by something? Or was it a Thing in itself?...What could there be about a shadow that was terrible that she knew that there had never been before or ever would be again, anything that would chill her with a fear that was beyond shuddering, beyond crying or screaming, beyond the possibility of comfort?

The Medium lost the delighted smile she had worn till then. "Oh, why must you make me look at unpleasant things when there are so many delightful ones to see?"
Again Mrs. Which's voice reverberated through the cave. "Therre will nno llonggerr bee sso many pplleasanntt thinggss too llookk att iff rressponssible ppeoplle ddo nnott ddoo ssomethingg abboutt thee unnppleasanntt oness."