Many words could be used to describe the mood in Edgar Allan Poe's brilliant poem "The Raven." In a single word, it can be considered "Gothic," which you can read more about in the link below. More specifically, however, the mood is mysterious, melancholy, and even morbid.
Mystery pervades the poem from the beginning to the end. At first there is a mysterious rapping that the speaker believes is someone tapping on his door, but when he opens the door, he sees only the dark night. The rapping continues, and he realizes it is now at the window. Opening the window, he is at first pleased at the surprise visitor that flies in, and he tries to guess how it may have come to him. When he asks it what its name is, and it responds, "Nevermore," however, he becomes even more intrigued and tries to imagine its background and how it came to be able to speak such a doleful word. When it speaks again, he begins "linking fancy unto fancy" and "guessing" about the bird. The reader shares the speaker's curiosity. He then begins ruminating about his lost Lenore, and the reader wonders about that relationship. Finally, in the last stanza, rather than being solved, the mystery continues as the reader wonders whether the speaker's soul ever will "be lifted" from out of the shadow of the raven.
The melancholy mood is set up by the loneliness of the speaker and the darkness of the night. When the speaker posits that the raven will leave him like "other friends have flown before," and as his hopes have flown before, we feel his sadness and despair. It isn't long before the speaker begins obsessing over the worst of those losses, namely his "lost Lenore." The very sound of those words together creates a moan that we can feel in the depths of our being. He longs for "respite and nepenthe," a potion that could make him forget his sorrow. The raven's consistent reply of "Nevermore" is itself a very melancholy word and concept, suggesting that whatever happiness may have once brightened the world will never be felt again. Finally, the final stanza creates a powerful image of melancholia, the term that in Poe's day was used for depression. The speaker's soul abiding forever, trapped beneath the shadow of "Nevermore," is a sad picture indeed.
The morbid tone of the poem comes from the speaker's focus on death, specifically Lenore's, but also his own and death in general. The word "Plutonian" speaks of Pluto, the ruler of the domain of the dead. Other words that refer to the afterlife are Aidenn, angels, and nepenthe. According to mythology, nepenthe was a drink offered to the dead as they crossed the river Styx into Hades so that they would forget their lives on Earth. In the fifth stanza, the speaker is so immersed mentally in thoughts of death that he believes the tapping at his door and window could be the ghost of Lenore; that is why he whispers her name.
The mysterious, melancholy, and even morbid mood of this poem have been haunting reader's since it was penned by the literary genius Edgar Allan Poe.