He is haunted by a demon, a demon against which he feels powerless, because in its first manifestation it has no face, no name, nothing; and the words, the poem he makes, are a kind of exorcism of this demon.
As a rule, said Holmes, the more bizarre a thing is the less mysterious it proves to be. It is your commonplace, featureless crimes which are really puzzling, just as a commonplace face is the most difficult to identify.