Which item does Poe continually personify?

Although the description of the Red Death in the first paragraph isn't personification, strictly speaking, it still gives the impression that the disease has intention or moves purposely. The narrator says that it "had long devastated the country," that it is not only fatal but "hideous." Moreover, "Blood was its Avatar and its seal . . . And the whole seizure, progress, and termination of the disease, were the incidents of half an hour." Such descriptions make it sound as though the disease is purposely grotesque and awful, that it intentionally moves quickly to devastate its victims. Likewise, when the narrator describes everything with which the prince has stocked the abbey, he says, "All these and security were within. Without was the 'Red Death.'" Again, this makes it sound as though the Red Death haunts the abbey, threatening, promising, to find entry.
The later descriptions of the Red Death, fully personified now, build on this impression. It is described as a "mummer" at the masquerade, its face and brow of its mask "besprinkled with the scarlet horror" of blood. It begins to move, once the prince sees it, "with a slow and solemn movement, as if more fully to sustain its role, stalk[ing] to and fro among the waltzers." When the prince chases the Red Death, the disease "turned suddenly and confronted his pursuer," and after the prince falls dead, the revelers attack the disease personified, only to see that the mask and costume are "untenanted by any tangible form." In other words, there is no body within the costume, and yet it has moved as though alive. Each of the prince's guests fall where they are, all dead.

Poe personifies the affliction of the Red Death, a fatal disease that strikes without warning and destroys its victims in thirty minutes' time.
Its first description through the use of personification is: "the figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave." Habiliment is an archaic noun that means clothing, so readers can envision a burial shroud. The narrator describes it as "stalking" among the people waltzing at Prince Prospero's party.
The figure is so intimidating that instead of seizing it, as the prince commands, the revelers shrink from it as it passes through the crowds. After the prince himself confronts, with a drawn dagger, the figure, the prince falls dead. At last, the revelers find the courage to seize the figure, but in doing so, they find that there is no physical form beneath the clothing.