Somnium is a multiverse linked by the power of a forgotten higher being.
The worlds are linked together through what is known as the "White space" and the "Black space"
The white space is generally considered to be safe to travel through, while the black space is dangerous and crawling with unspeakable horrors
What makes the white space safe to travel through, are powerful beings known as the "warp" specifically in this case, they are the warp guides.
There are several dimensions of somnium that are seperate from eachother.
For example, there is Nightmaria, oceana, galax, lala, and mt.infinity.
Many of these worlds harbor odd traits.
Oceana is an infinite ocean, Nightmaria seems to blur the concept of life and death , along with harboring many horrors, and mt.infinity is an infinite mountain that has several subdimensions within it that represent differences in height as you "climb"