How does the song in Edgar Allan Poe's "The Fall of the House of Usher" relate to the characters?

The song, entitled "The Haunted Palace", echoes the degeneration of both the house of Usher and its inhabitants.  The palace in the song was a beautiful and fertile place that was dominated by intellect, where everyone lived in peace.  The house of Usher, as the narrator tells us, was once a respected home of a respected family and was beautiful in its day.  It is now a shell of its former self, overgrown and breaking down, with only two people inside, both ill.  However:

But evil things, in robes of sorrow,
Assailed the monarch's high estate;
Shall dawn upon him, desolate!)
That blushed and bloomed
Of the old time entombed.

The "robes of sorrow" have taken over Roderick Usher, who called the narrator to him for help with a nervous condition.  Instead of intellect dominating, fear and grief dominate in the household.  There is no existing in harmony because the residents are not at peace themselves.

In the last stanza, there is a reference that can be connected to Madeline:

Vast forms that move fantastically

She is a form that moves fantastically - almost ghost-like - throughout the house.  We can assume that the feeling of disquiet and/or grief has overcome her, too, and that her weakening is further enflaming Roderick's. 

With this song, Poe is both symbolizing and foreshadowing the situation in the "House of Usher".