In one sense, the masked figure who crashes Prince Prospero's party is representative of the disease -- called the Red Death -- that ravages the kingdom. The disease is referred to as the Red Death because it is characterized by the hideous quantities of blood that seem to pour out of a person when they have contracted it. The narrator says of the disease, "Blood was its Avatar and its seal -- the redness and the horror of blood" because its victims would literally bleed from all their pores before they died. The masked figure comes, having "assume[d] the type of the Red Death." His clothes are "dabbled in blood -- and his broad brow, with all the features of the face, was besprinkled with the scarlet horror." He is the disease, personified.
However, the masked figure is more than just the disease. He can be interpreted as a symbol of death, in general, too. His mask looks like the face of "a stiffened corpse," and he is first seen by Prince Prospero as he stalks through the seventh room of black and red. This is the only room to be characterized by two colors, and so they both must be significant. Just as red is associated here, with the fatal disease, so black is often symbolic of death in general. Likewise, clocks are often symbolic of mortality and death, and so the ebony clock in this room provides another clue that anything associated with the room is likewise connected to death. It is not the Red Death, the disease, that holds "illimitable dominion over all" but rather death in general. No human, no matter their status, can escape death, though the prince clearly thought that he could. Thus, the masked figure represents both the bloody disease as well as death, generally speaking.