Anything phantasmagorical feels or looks like a crazy dream. If you see a psychedelic music video that looks like it was shot through a kaleidoscope, you can call it phantasmagorical.
A phantasmagorical film is surreal and strange, rather than straightforward and literal (think the Beatles' "Yellow Submarine"), and phantasmagorical paintings are equally full of odd, impressionistic images, colors, and designs. If you have a phantasmagorical nightmare, it might be hard to describe to another person, it's so weird and fantastic. The adjective phantasmagorical comes from phantasmagoria, the name given to a "magic lantern" show in 1802, from the Greek phantasma, "apparitition."