The Red Death shows up at the masquerade ball at midnight, sharp.
We have no idea where he's come from, or how he got in. We don't even know that he's the Red Death yet. So far as we know he's just some guy dressed up in a Red Death corpse costume. Everybody else thinks this appearance is inappropriate.
The Red Death stalks around. Prince Prospero demands that he be seized and unmasked, but no one has the guts to follow the order.
The Red Death is unfazed and walks right by Prospero, making his way through the suite towards the black room. When he reaches the violet room, Prospero dashes off after him in pursuit, dagger drawn.
At the edge of the black room, the Red Death whirls around to confront Prospero, who promptly falls on the floor and dies.
A whole horde of other guests jumps the Red Death, but find when they rip of his mask and costume that there's nothing underneath. The guy in the Red Death costume is actually…the Red Death.
Everybody dies, the lights go out, and the Red Death holds dominion over all.