The Rain Man is an entity that either is meant to be the entity Baal, which is just a word that means Lord but applied to gods worshipped in the Levant region most prominantly associated with the storm and fertility god Hadad of the Canaanites. Over the years the name Baal has become demonic in nature after the emergence of Christianity often being depicted as just one demon of hell or another name for Satan.
The Rain Man is either Satan, Baal or a seperate entity that acts sort of a middle man between man and Satan or Baal.
The Rain Man conspiracy came about in the 2000s when people began connecting lyrics such as Eminem’s Rain Man and Rihanna’s Umbrella and forming a conspiracy that still persists in some circles, sort of a remnant of the satanic panic the idea that famous musicians work for the devil is nothing new but this Rain Man angle is fairly recent and a product of the 21st century.
The idea is that this entity rains down wealth and fame upon a person in exchange for servitude and allusions in their work to satanic imagery or to rain man himself.
Notes
- To Sufferers from Nervous Depression (1869) here only for illustrative purposes of what the Rain Man may look like.