The Masque of the Red Death
How we cite our quotes:
The figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave. The mask which concealed the visage was made so nearly to resemble the countenance of a stiffened corpse that the closest scrutiny must have had difficulty in detecting the cheat. And yet all this might have been endured, if not approved, by the mad revellers around. But the mummer had gone so far as to assume the type of the Red Death. (9)
Prospero and his friends have just been reminded about the very thing they were trying to forget: the Red Death. Somebody's had the nerve to dress up as a corpse killed by the disease. Death has infiltrated their secure world. They don't yet know quite how literal that is.
…when the latter, having attained the extremity of the velvet apartment, turned suddenly and confronted his pursuer. There was a sharp cry --and the dagger dropped gleaming upon the sable carpet, upon which, instantly afterwards, fell prostrate in death the Prince Prospero. (13)
Prospero succumbs to death. His attempts at avoiding it have proven useless. The irony is that he literally charged right into it. And fittingly, he dies right at the threshold of the "death" room.
And now was acknowledged the presence of the Red Death. He had come like a thief in the night. (15)
Now it's revealed that Death itself is actually present at the masquerade. The "thief in the night" bit probably sounds familiar, and that's because it's an allusion to a famous Biblical passage. Poe compares death to Christ at the time of the last judgment. He comes for everyone, when they're least expecting it, and his judgment cannot be avoided.