The intruder is the Red Death itself. In the next to the last paragraph of the story, the revelers surround the masked figure and take off its mask. Poe writes that they " gasped in unutterable horror at finding the grave-cerements and corpse-like mask which they handled with so violent a rudeness, untenanted by any tangible form." The word "untenanted" means "uninhabited". So when they took off the mask, they found nothing underneath. Poe then writes, "And now was acknowledged the presence of the Red Death." Then each of the revelers dies showing symptoms of the disease. Poe says they died "in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall." This mirrors Poe's description of someone suffering from the Red Death at the beginning of the story.