The word that best describes the mood in Edgar Allan Poe's "The Raven" is "uncanniness." The word "uncanniness" describes a feeling of fear combined with wonder aroused by something potentially dangerous that has never been experienced before: mysterious; arousing superstitious fear or dread."
The speaker doesn't know what to make of this black bird that pecks for admittance to his lonely chamber and then makes itself at home by perching on top of a bust of Pallas Athene (Greek goddess of wisdom) above his chamber door. He doesn't know whether to be frightened or amused. He doesn't know whether the bird is an evil spirit or just a pet that escaped from its owner's home and is seeking shelter from the storm in another human domicile.
The bird keeps repeating the single word "Nevermore," but the speaker doesn't know whether the bird understands what it is saying or simply learned that one word from its "unhappy master." The dictionary definition of the adjective "Uncanny" is "Exciting wonder and fear; inexplicable." The fact that this bird can speak at all makes it all the more uncanny.
The speaker is left in deep despair. The reader is left wondering whether the bird was a supernatural messenger from the world of the dead or just an ordinary raven caught by some human and taught to say a single word. The mystery is like the mystery of life itself. Is there life after death? Is there any possible hope in the promises made in the Bible? The speaker asks the bird that question, half hoping for an answer.
Tell this soul with sorrow laden if, within the distant Aidenn,
It shall clasp a sainted maiden whom the angels name Lenore--
Evidently the bird doesn't know the answers any more than the speaker but it merely represents the mystery itself.