Sheeted Memories of the Past— (line 34)
We run into these guys right after the ghouls, but the feeling is a little different. Now we're not just talking about creepy crawly creatures, but about ghosts that actually mean something to the speaker. This starts a change in the poem. Before we were dealing with the kind of scary stuff you might find in a horror movie or a video game, but now the supernatural world starts to connect with the speaker's feelings of sadness.
White—robed forms of friends long given, In agony, to the Earth— and Heaven. (lines 37-8)
Now we get to the heart of it. The speaker is grieving for his lost loved ones. If you've read any other Poe stories or poems, this should be a super-familiar theme. In a way, this is the key to the whole poem. The speaker didn't just wander into Dream-Land to see the sights, and he's not just bummed out for no reason. He wants to find these ghosts, since he can't see them in the real world any more. Seeing these spirits in their white robes is comforting, it soothes him in his sadness. All of sudden the supernatural parts of this poem start to seem weirdly happy and almost beautiful.