In both tales, the dominant effect of horror is achieved in part because the form of death comes through a particularly horrible disease that the story's characters dread.
In "The Masque of the Red Death," the Red Death is made horrible by severe pain and the "profuse bleeding at the pores." Likewise, Madeline in "The Fall of the House of Usher" has a mysterious and frightening wasting disease that displays "a partially cataleptical character." In other words, the emaciated woman, no more than a skeleton, sometimes appears to be dead when she is not.
In "The Masque of the Red Death," the thousand healthy guests Prince Prospero has invited into his walled and locked castle try to push out of their minds their ghastly fear of the Red Death through dancing and partying in beautiful rooms. However, their merriment is interrupted every hour by the tolling of a large ebony clock, which endlessly reminds them of their mortality.
In "Usher," Roderick likewise tries to bury his terror at his twin sister's disease and imminent death in music and art, but his nerves are ever on edge.
The menacing threat of death hangs over each story, creating anxiety and setting us up for the final moments, in which catastrophe and death do arrive to each setting.