The double entendre of the title is repeated at the end of Poe's "The Fall of the House of Usher." That is, the close parallels of the family and the mansion end in phantasmorgia as "the full, setting, and blood-red moon now shines through the fissure of the house". This eerie glow of dying life symbolizes the tragic flaw of the Usher lineage.
Roderick Usher dies of fright when Madeline falls upon him in the throes of death. The narrator flees in terror at the destruction of both the mansion and the genetic lines of the Ushers. Madeline, Usher's identical twin, comes to claim him; perhaps she represents a part of Roderick's mind that he has tried to suppress, some dark urge or family secret. Certainly, the lasting image of the "blood-red moon" and the collapse of the Usher mansion--
a long tumultous shouting sound like the voice of a thousand waters--
as it is swallowed by the mountain lake suggest the tragic end of the Usher family, a family plagued by disturbances of the mind, "fissures" of rational thought.