The 50th Dead World is a world that resembles Heaven for ghost. This is where
all the spirits go once they find out their cause of death. So far the only known people to have gone to the 50th Dead World is Ghost Princess and Clarence. If they don't, they stay in the Land of Ooo as an empty soul. This has been mentioned first in the episode "Ghost Princess." Nothing is really known about it, however, just the fact that ghosts who have found their cause of death are ascend into the 50th Dead World. No one knows why there is 50 dead worlds, and all that is really known is that the 50th Dead World is Heaven for ghosts.
The entrance seems to be filled with blue smoky lights. It seems to also lead to a blue-like world.