Dr. John Markway narrates the history of the 90-year-old Hill House, which was constructed by Hugh Crain as a home for his wife. She died when her carriage crashed against a tree as she approached the house for the first time. Crain remarried, but his second wife died in the house from a fall down the stairs. Crain's daughter Abigail lived in the house for the rest of her life, never moving out of the nursery. She died calling for her nurse-companion. The companion inherited the house, but later hanged herself from a spiral staircase in the library. Hill House was eventually inherited by a Mrs. Sanderson, although it has stood empty for some time.
Markway wishes to study the reported paranormal activity at Hill House. He secures a lease from Mrs. Sanderson to occupy the mansion for the duration of his investigation; conditional to his acceptance is that he take Luke Sanderson, her heir, with him. Markway has chosen two individuals to accompany himâa psychic, Theodora, also known as Theo, and the meek Eleanor Lance, who experienced poltergeist activity as a child. Eleanor spent her adult life caring for her invalid mother, whose recent death has left Eleanor with severe guilt.
The large, maze-like mansion's walls were constructed with angles slightly askew, resulting in off-center perspectives and doors that open and close by themselves. The immense library contains the ramshackle spiral staircase from which the previous owner hanged herself; the vast conservatory is adorned with eerie statues. During their first night in the house, Eleanor and Theo are terrified by supernatural occurrences outside Theo's bedroom door. Deafening banging is heard against the door and the voice of a young girl is heard echoing with laughter. Despite the turbulence, Eleanor feels a tentative affinity to Hill House.
The team explores the house the next day, discovering a cold spot outside the nursery and encountering other supernatural phenomena. Markway reveals more about the hauntings that have allegedly occurred. Following another night of loud disturbances the team discovers the words "HELP, ELEANOR, COME HOME" on a wall, which causes Eleanor severe distress.
That night, Theo moves into Eleanor's room and they fall asleep in the same bed. Eleanor is awakened by the sounds of a man speaking indistinctly and a woman laughing. Fearful, Eleanor asks Theo to hold her hand and she feels it being crushed. As Eleanor hears the sound of a young girl crying, she shouts at whoever is causing the child pain. Theo awakens with a start and turns on the light. Eleanor then sees that she has moved from the bed to the couch, and realizes that Theo was not the one she felt holding her hand.
The following day Dr. Markway's wife Grace arrives at Hill House to warn her husband that a reporter has learned of Markway's investigation of Hill House. Markway is concerned when Grace announces that she plans to join the group for the duration of the investigation. She demands a bed in the nursery despite her husband's warning that it is likely the center of the disturbances. That night the group experiences loud banging and an unseen force attempting to force its way through the living room in which they are staying. The banging then proceeds to move its way up towards the nursery, where the sounds of it destroying the room are heard. This prompts Eleanor to run towards the source; however Grace is nowhere to be found. Her disappearance is then confirmed the following morning. Eleanor's mental instability worsens as she falls further under the spell of Hill House. She enters the library and climbs the dilapidated spiral staircase. Once she reaches the top, Grace appears unexpectedly at a trap door and the startled Eleanor nearly falls to her death. Markway rescues Eleanor but just misses seeing Grace, who has disappeared back into the house.
Markway becomes alarmed at Eleanor's obsession with Hill House in spite of the dangers it poses for her. Despite Eleanor's pleas to stay, Markway insists that she leave at once and asks Luke to accompany her home. Before he can join her in the car, Eleanor drives off and speeds down the road toward the front gates. She soon feels the steering wheel move by itself and the car advances erratically. At first she struggles to regain control but then surrenders to the unseen force. Suddenly Grace appears from behind a tree and steps in front of the car. Eleanor crashes into the tree and is killed. Luke observes it seemed that Eleanor deliberately aimed the car at the tree, but Markway asserts that something was in the car with her. He notes that the tree that claimed Eleanor's life is the same one that killed the first Mrs. Crain. Theo remarks that Eleanor got what she wantedâto remain with the house.

Question: What caused Eleanor's mental instability to worsen?
Answer: The overall experiences at the house caused Eleanor's mental instability to worsen.