“It is a sickness which has no cure.”
The Nightmare Region was created in the aftermath of the War of Black Fog after Shu'rimrodir was imprisoned within the Realm of Dreams. Its existence is attributed to Shu'rimrodir's Nightmare Energy contaminating the Dream Energy of the Realm of Dreams and slowly converting it into a brand new plane controlled by the Corruptus lord. While at first insignificant in size, the Nightmare Region eventually expanded without control. Mortals and immortals alike were linked to it through their souls and began suffering from nightmares. This link with the rest of reality turned the Nightmare Region indestructible.
The Nightmare Region is a realm composed entirely of Nightmare Energy. In a contrast to the Realm of Dreams, which appears as an abstract dreamy version of the person's memories, the Nightmare Region takes the form of the person's worst fears, phobias and nightmares. The Nightmare Region is infinite in size, but it is visible from inside the Realm of Dreams. Seeing it from the outside makes it appear much smaller than it actually is.
To access the Nightmare Region, one needs to first access the Realm of Dreams to get to it. As the Region is completely composed of Nightmare Energy, it is considered incredibly hazardous for unprotected beings. It is also highly toxic to Dream Energy beings, though powerful enough users can enter the Region and create Essence barriers to protect themselves.