Haunted by ill angels only, (line 2)
This helps to set the ghostly, mysterious tone for the whole poem. By this point, we don't have any idea where we are going, we just know that it's dark, lonely, and we are surrounded by bad spirits of some kind. "Ill" is a great word to pick here, because it can mean both "sickly" and "evil." Using that word to describe angels is confusing and unexpected, since we usually imagine angels as healthy, shining, full of goodness and light and all that stuff. Poe lets us know right away that we're not in Kansas anymore. Not only is this world full of spirits, they are sick and evil too.
Where an Eidolon, named NIGHT, On a black throne reigns upright, (lines 3-4)
Now we get even weirder. This land we've just come to us apparently ruled by a phantom (an "Eidolon") called "NIGHT." Poe tells us just enough about this mysterious character to make our spines tingle. We can sort of picture him sitting on his black throne, and we can almost see his rigid, straight ("upright") body. Yet there's still some mystery here. Part of creating a scary effect in a poem is not telling your reader too much. You want to keep them guessing, make sure that they know enough to be curious but not enough to feel like they have everything figured out. Poe's the master of this kind of scary poetry.
Where dwell the Ghouls,— (line 30)
In Arab legend, ghouls were hideous monsters that fed on corpses. With this short, sharp line, right in the middle of the poem, Poe is definitely trying to grab our attention, to draw us deeper into his creepy world. "Dream-Land" is crawling with ghostly creatures, and these seem like the worst of all. We only see these creepy, flesh-eating swamp-dwellers for a moment, but that's long enough to make your skin crawl.