One definition from Dictionary.com describes phantasmagoria as "a shifting series of phantasms, illusions, or deceptive appearances, as in a dream or as created by the imagination." Throughout history this definition of phantasmagoria developed as a form of horror theater produced by a magic lantern to project frightening images onto walls, smoke, or screens that increase or diminish in size, pass into each other, or dissolve. Many of the shows began as séances in Germany in the late 18th century, and as they gained in popularity, became a form of entertainment. As the concept grew, the proprietors of these shows developed new techniques and evolved into trick films that used transformations, superimpositions, disappearances, and rear projections. Many modern day horror films use elements from these show.
The Wikipedia article tells a fascinating history of people's interest in the gods, spirits, and particularly the dark arts and conjuring of things from beyond. It calls to our inner curiosity and our attraction to the unknown and the macabre. Michael from Vsauce has a wonderful video, which you can watch to the right, that describes this unique and paradoxical aspect of our human nature.
This piece makes heavy use of chromatic mediants to give it a constant feeling of ambivalence in tonal center, creating a sense of mystery and fascination in the unknown. The key centers shift several times throughout the piece, just like the phantasms created by the projections, and it's chromaticism creates a feeling of awe and wonder but with a concealed, dark undertone, beckoning to this part of our primordial brains. It's that desire to figure out, "What was that noise?" when our more sensible self tells us otherwise.