what are the vast forms that move fantastically?

Poe relies heavily on personification to set mood and tone. There isn't a particular list of forms that move rather Poe leaves this up to our imaginations. Poe describes the eye-like red-litten windows "seeing vast forms that move fantastically to a discordant melody". These "forms" personify the discord of both the house and the Usher family. They are forms of the tormented Usher family, the house becoming a manifestation of the slow insanity that has taken over.