Any novel titled The Haunting of Hill House is bound to have fear as one of its main themes. When Eleanor and the rest of the investigative team arrive at the notorious Hill House, they certainly weren't expecting to find a house that could be described as "normal." There are all sorts of things waiting to scare them at Hill House, including doors that refuse to stay open, spots in the house that are inexplicably icy, something akin to a dog that is seen running around, and a room that gets splashed with blood. In a nutshell, there is no shortage of reasons for Hill House's visitors to be alarmed.
Eleanor is a fascinating character in this story, and I would argue that her biggest fear is not Hill House or any spooky thing that it may be able to throw her way. For Eleanor, the most frightening thought in the world would be going back to her old life, in which she was nothing special and in which most of her time had been spent nursing her invalid mother, whom she hated. Coming to Hill House had signaled the start of a new life for the introverted Eleanor, and as her suicide at the end of the novel shows, leaving Hill House is a far scarier prospect than anything she encountered within those crooked walls.