What happens after the mysterious figure is unmasked?
When the revelers descend on the Red Death in a rage, following the collapse of Prince Prospero, they snatch at his robes and find that the costume is "untenanted by by any tangible form." In other words, there is no person, no body inside the robes. It is at this moment that the masqueraders realize they are in the presence of the terrible disease that has decimated the kingdom. As soon as they recognize this, the revelers begin to drop, one by one, and each of them lands in the "despairing posture of his fall." The narrator says that the black clock in the hall stops ticking the moment the final reveler dies; this clock has reminded the party-goers of their own mortality over and over again, every time it chimed the hour. Therefore, it makes sense now that it ceases to keep time when the revelers are literally out of time. Finally, the flames in the hall go out. There is nothing left now besides the Red Death.
After the death of Prince Prospero in pursuit of the mysterious figure of the unknown guest, the other revellers grab hold of it only to find there is nobody underneath the mask and grave-like shroud. This increases the general sense of horror that its presence has produced, and the revellers all drop dead in turn. The mysterious figure is the plague personified. Prince Prospero has sought to keep it out, but no-one even noticed when it got in. The whole story is full of symbolism. For instance there is the colour symbolism of the seven rooms. The grim seventh room, with its blood-red glare on the windows and velvet blackness obviously stands for death, and the ominously ticking clock symbolises how time eventually runs out for everyone. The story shows how no-one, try as they might, can ultimately escape death.