But evil things, in robes of sorrow, (33)
These nasty things come out of nowhere. They kind of make us think of the Dementors in Harry Potter, or maybe the Ringwraiths in the Lord of the Rings. We imagine long, flowing black robes, horrifying ghostly faces, etc. Whatever they look like, they show up in this poem to let us know that the party is over. The happy spirits and good angels are permanently gone, and the "evil things" have taken their place.
Vast forms, that move fantastically (43)
These "vast forms" are the dark-side mirror image of the spirits we met in line 19. Where those spirits moves "musically," these ones move "fantastically." There's something disturbing and grotesque about the change. It's just one more instance of spirits helping to make clear the central metaphor of madness and decay that runs through this poem.