[Poetry speaks in many different ways to a vast number of individuals based upon each's person's experiences. As with any art form, once the art is released to the world, it takes on a life of its own. These are only my opinions.]
Victor Hugo was a writer of such famous works Les Miserables and The Hunchback of Notre Dame.
A recurring theme in Hugo's work is humanity's ceaseless combat with evil.
Hugo was also a poet. In that he wrote of the conflict within man—a battle between good and evil—it is not surprising that he wrote "Et nox facta est"...
Among his most ambitious works was an epic poem, "Et nox facta est," ("And There Was Night"), a study of Satan's fall. The poem was never completed.
The poem, "Et nox facta est" from The End of Satan (La Fin de Satan) refers to the fall of Satan when he (and his "host") was cast out of the heavens for challenging God's authority and power, intent upon building his own "kingdom." The far "fall" described in the poem expresses the passage not only through time but of space (distance) as well.
For four thousand years he fell into the abyss.
This might also indicate the magnitude of the separation of God from evil or sin. As Satan was once an angel, when the poem speaks of his feathers, I assume they come from an angel's wings. As Satan falls, the feathers from his wings fall, too.
The feathers from his wings fell more slowly behind.
The feathers, literally, may simply refer to the physical deterioration of Satan's wings. Figuratively, the slower falling wings may refer to the change Satan goes through in leaving God's presence—theologically, this is often offered as a perception of hell: the absence of God. Hugo might imagine that Satan's separation from God could be seen in the fallen angel's loss of "heavenly adornment" and power. The feathers falling might point to Satan's inability to rise to the lofty heights of heaven because of his sin—there is no way for him to approach God's presence even to ask for forgiveness, as he has fallen so far and no longer has the ability to rise up again.
It might be a stretch, but consider the mythological story of Icarus. His father, Daedalus, made two pairs of wings held together by wax so they could try to escape from the island of Crete. In flying too high (out of curiosity and excitement), the wings melted, and Icarus plummeted to the earth and died. Perhaps the feathers in the poem allude to Icarus and his foolishness—Hugo may be referring in his poem to Satan's foolishness: his ego and "delusions of grandeur."