In the final stanza, what images suggest that the palace is now haunted by madness?

The last image the Narrator describes seeing is that of the House of Usher splitting apart along the previously noted zig-zag fissure. The walls are bursting and the fragments are swiftly disappearing into the "deep and dank tarn."

The storm was still abroad in all its wrath as I found myself crossing the old causeway. Suddenly there shot along the path a wild light, and I turned to see whence a gleam so unusual could have issued; for the vast house and its shadows were alone behind me. The radiance was that of the full, setting, and blood-red moon which now shone vividly through that once barely-discernible fissure of which I have before spoken as extending from the roof of the building, in a zigzag direction, to the base. While I gazed, this fissure rapidly widened--there came a fierce breath of the whirlwind--the entire orb of the satellite burst at once upon my sight--my brain reeled as I saw the mighty walls rushing asunder--there was a long tumultuous shouting sound like the voice of a thousand waters--and the deep and dank tarn at my feet closed sullenly and silently over the fragments of the "House of Usher".