The Na’Kall are beings that exist between realities.
The Na’Kall tend to the pathways that exist between worlds and link them all together. They serve as protectors of existence. When sentient beings dream they connect to other realities like a computer network. When sentient beings sleep they are sharing the cognitive load with that reality and allowing their mind to function at a lower level.
When sentient beings like humans take control of their dreams and begin to travel through or influence the worlds they are seeing the Na’Kall come to end the beings lives. It is important that only beings who understand the complexity of the connections between realities influence them and so the Na’Kall have become a race of nightmare creatures lurking in the dark parts of our minds and waiting for us to slip up.
Most people don’t realize what they are doing when they dream. For those who do remember and influence the other realities the travels become addictive. Their minds no longer contribute to the connections of the universe but instead they begin to destroy the very fabric of the dimensions they influence.