This theme doesn't show up at all until the third section of "The Bells." From that point on, though, it really takes over. Poe explores the sounds and the feelings of fear. He uses all kinds of words to explore the different ways that people can be scared. He spends a lot of time dealing with these kinds of dark emotions, which makes sense because he's definitely one of the grandfathers of the horror genre.
"The Bells" is about the careful balance between terror and joy. The two emotions are intertwined in the poem, and they can never be fully separated.
The poem's speaker slowly draws us down into a nightmare of insanity and fear, from which we are unable to escape.