The action in chapters one through eight of Mary Shelley's Frankenstein varies greatly. In the opening chapters, excluding the letters, Shelley allows Victor to introduce himself to the readers. Victor tells readers of his family's background and of his initial interest in science. It is not until chapter four that the action of the story takes a horrific twist.
In chapter four, Victor describes himself visiting graveyards and charnel houses (tombs) in order to collect "supplies" for his experiment. He wishes to reanimate life and is gathering body parts in order to put life back into them. In chapter five, Victor finds success. His creature awakens, looking at him with yellow eyes and black lips. Given the limited nature of Shelley's description, reader's imaginations are allowed to "run wild" (to be unrestrained). The image which readers construct are, essentially, limitless in regards to horror. It is up to each reader to figure out how horrendous the creature is (or is not). This certainly adds to the feeling of terror for many readers.
In chapter seven, the death of William is described. The mark of death on his neck, readers are openly told that the young boy was murdered. Readers, and Victor, assume the creature is responsible. This adds to the terror of the tale. What more can the creature do? Chapter eight adds to the horror. Justine has been charged with William's death, and readers know that she is not responsible. Justine dies for being found guilty. This can be terrifying for some readers given her lack of guilt.