From beginning to end, the tone of "The Masque of the Red Death" is grave, as in dread-inducingly serious. It's ominous: you never quite escape the sense of a looming threat. And it's plenty dark. Poe sets the tone right away – just look at the opening lines:
The "Red Death" had long devastated the country. No pestilence had ever been so fatal, or so hideous. Blood was its Avatar and its seal --the redness and the horror of blood. (1)
See what we mean? It's as if the whole story were written to be read aloud by one of those slow-speaking, deep-voiced old guys who always pops up in the movies to issue dire warnings like "If he is not stopped, he will bring the greatest destruction the world has ever known."
Then again, there's also all that dream imagery, like: "And these – the dreams – writhed in and about, taking hue from the rooms, and causing the wild music of the orchestra to seem as the echo of their steps" (7). At those moments the narrator himself seems to be caught up in the dizzying whirl and delirium of the masquerade he's describing.