The Heavy Rain Region is a zone within the Area 8 Mainland where heavy rain of different kinds fall. The rain that falls here is very dangerous and hence the place is uninhabitable. The Heavy Rain Region is divided in four main regions which are the Stardust Hill, Poison Rain Prairie, Beehive Plain and the Nightmare Hill with smaller unmarked areas being shown (possibly parts of the main region).
The Nightmare Hill is the only place where rain doesn't fall however it is still a dangerous region as it is the nest of the Horse King Heracles. The Heavy Rain Region is shown as an ocean surrounding the Slow Rain Hills but is just a region where torrential rain falls.