Using word choices, setting, ghosts and a mysterious errand, the author makes this poem very uncanny. He describes how there is a "moonlit door," so, it is at night, and it is so quiet that you can hear "his horse" as he "chomped the grass" in the woods. The traveller is "perplexed", so the author makes him confused and anxious, which takes the mystery and tension up a notch. Then, there is the uncanny "host of phantom listeners" that "stood thronging the faint moonbeams on the dark stair", which indicates that there is a large number of ghosts that are crowded in the house, in the moonlight, listening to him. Uncanny means creepy, eerie, or mysterious, and that description is definitely horrifying. Imagine not just one, but an entire "host" of ghosts just waiting and listening. It is a dramatic moment, and the mystery is just deepened as we wonder who they are and what they are going to do. It describes that the traveller "felt in his heart their strangeness," so, the man is aware of them on some level, kind-of like how the hair on your neck prickles when you sense someone in the room with you. That makes it even more dramatic, because he isn't oblivious, but knows something is up. The mystery is deepened as the author adds the dramatic dialogue from the traveller: "Tell them I came, and no one answered, that I kept my word." This mysterious, ambiguous and intrigiung phrase makes us wonder who are "they"? Where was he? What did he do? Why did he have to come back and tell them? It is a puzzle, and quite a mystery. We then wonder what the strangers will do, but they just "surged softly backward" as he leaves.
The entire poem is very uncanny and mysterious; the author uses word choices, an eerie setting, a host of ghosts, and a mysterious quest from the traveller to enhance that mystery. I hope that helped. Good luck.