The Unknown (5e Environment)
From D&D Wiki
A place deathborne forms go when they are done with their current body. A portal of pure black and purple tears itself into reality, the humans back will rip open for the final time, and the deathborne will be uncontrollably pulled in. Many have tragically disappeared trying to explore the Unkown. In fact, no one has ever mapped the Unknown.
Human eyes cannot see the darkness of the environment. Those with darkvision can see through the endless darkness, which reveals sinister spirits dancing, and zooming around. From the moment one enters this plane until they leave, a person would be in constant torment. For every minute they remain there, they will suffer psychic damage of 1d12.