I actually had an occasion to use a derivative of this word
in a sentence.
She shut her eyes and willed the picture of the wagon crushing his body to escape from her head. Instead, it returned, replaying in a most horrible phantasmagoric fashion.
That sentence for a book I'm writing.
1. a shifting series of phantasms, illusions, or deceptive appearances, as in a dream or as created by the imagination.
2. a changing scene made up of many elements.
3. an optical illusion produced by a magic lantern or the like in which figures increase or diminish in size, pass into each other, dissolve, etc.
It has an interesting word history.
1802, name of a "magic lantern" exhibition brought to London in 1802 by Philipstal, the name an alteration of Fr. phantasmagorie, said to have been coined 1801 by Fr. dramatist Louis-Sébastien Mercier, from Gk. phantasma "image" + second element probably a Fr. form of Gk. agora "assembly" (but this may have been chosen more for the dramatic sound than any literal sense). Transf. meaning "shifting scene of many elements" is attested from 1822.
Interestingly enough, "phantasmagoria" has its own Wikipedia entry
Phantasmagoria...was a form of theatre which used a modified magic lantern to project frightening images such as skeletons, demons, and ghosts onto walls, smoke, or semi-transparent screens, frequently using rear projection. The projector was mobile, allowing the projected image to move and change size on the screen, and multiple projecting devices allowed for quick switching of different images. Invented in France in the late 18th century, it gained popularity through most of Europe (especially England) throughout the 19th century.