This is the first major image in the poem. Longfellow really wants us to focus on the exact time of day, that moment where day shifts into night. To drive this home, he makes "Night" into something like a character in this poem, and stages a big entrance for him.
Line 2: In this line, darkness falls down "from the wings of night." We won't get the rest of this image until the first stanza is over, but right away you get the picture, right? Night is turned into a giant bird, with huge dark wings. When you describe a concept like night using words that usually apply to living creatures, we call that personification.