Victor receives a letter from his father telling him to return home immediately because William, Victor's youngest brother, has been murdered by strangulation. The significance of the letter is that it is a turning point in the book, in which the monster now has a real presence in the story; he is a threat to his creator. Up to this point, the monster has been in the back of Victor's mind. Here the monster is asserting himself into Victor's life.
The setting and atmosphere sets the stage for Victor to see the monster. The reader expects something to happen because of how Shelley describes the scene. Also, it is interesting that lightning has such a recurring role in the story. It is first how Victor learns about electricity and gains an interest in natural sciences. Then, it is assumed that he uses electricity to "jumpstart" his creation's life.
Not only does Victor know that the monster is responsible for the death of his brother (psychic communication), but the monster was able to find Victor's family (via psychic communication). Psychic communication is a Gothic novel quality.
