The Eagle is introduced as another way of thinking of the night. As far as important poetic symbols go, the eagle is way up there. For thousands of years, poets have used this bird to represent power, strength, nobility… all that good stuff. So, when an eagle sweeps in at the beginning of this poem, it carries a lot of weight. Think how different it would be if Longfellow had picked a pigeon instead.
Line 4: Here Longfellow is comparing the way that darkness rolls down the sky to the way a feather drifts down from the wing of an eagle. We call that kind of comparison a simile. He's using this comparison to make us focus really intensely on the moment of twilight. It's this change in light that launches the whole poem, so he rolls out some big poetic tricks to give it extra impact.