The 'fall' refers to both house actually caving in on itself and being destroyed and that the both last living descendants of the family name Usher (Roderick and Madeline) die.
No relation
After Roderick and Madeline die, the narrator escapes the house just as it begins crumbling down to the ground.
The house in "The House of Usher" was decaying and eventually collapsed at the end of the story killing Roderick Usher and Madeline. ~KKMG1
THe narrator has been invited by his old childhood frined Roderick Usher to come to the Usher house to try to cheer him up.