Final answer:
The nightmare description in paragraph 12 contributes to a foreboding and tense tone through vivid description and imagery, creating an atmosphere of dread that heightens the story's suspense.
Step-by-step explanation:
The description of the nightmare in paragraph 12 primarily affects the tone of the story by adding a sense of foreboding and tension. This is achieved through the use of vivid description and imagery that evoke a feeling of fear and unease. By describing how the silence of the waning afternoon 'breathed hateful portent' and 'terrified Jane,' the tone becomes dark and ominous, setting up an atmosphere of dread that permeates the narrative and engages the reader's emotions. Such descriptions serve to intensify the plot towards a climax, as the growing dangers and perils become more tangible and imminent in the character's mind.