What is the ancestral curse in "The Fall of the House of Usher" by Edgar Allan Poe?
The Ushers' ancestral curse is madness.
It is not entirely clear what has caused the Usher family's ancestral curse. Obviously, both the brother and sister suffer from something the doctor cannot identify, and it seems to be both physical and mental. It has to be genetic, and usually, when a genetic disease is passed down through a family to this extent, there is some inbreeding involved. In those days, families often tried to keep their lines “pure” by intermarrying, but the resulting lack of genetic diversity doomed them.
Roderick seems to have suffered under the knowledge and fear of his family’s ancestral curse for some time. Seeing his sister deteriorate eats away at him, and his mind can’t really take it. He asks his friend, the narrator, to visit him. The narrator becomes a witness to Roderick’s decline into madness.
Having seen his family's curse, Roderick fears his future.
I dread the events of the future, not in themselves, but in their results. I shudder at the thought of any, even the most trivial, incident, which may operate upon this intolerable agitation of soul. I have, indeed, no abhorrence of danger, except in its absolute effect—in terror.
When the doctor says Madeline is dead, Roderick loses it. He has her entombed in the cellar, and tells the narrator she is not really dead. He fears her, though. When she comes back to life, he is losing his wits. Her reappearance scares him to death—literally.
For a moment she remained trembling and reeling to and fro upon the threshold—then, with a low moaning cry, fell heavily inward upon the person of her brother, and in her violent and now final death-agonies, bore him to the floor a corpse, and a victim to the terrors he had anticipated.
The narrator has had it. With that, he flees from the house and its crazy undead inhabitants and their ancestral curse. He can't fully describe how horrible that house was or what he saw in it.