From Labyrinthe Wiki
Secundus hangs slightly out of phase with Primus, a kind of cosmic sump, where those things which did not have the strength to remain on Primus or have been believed into being without the power of a principle plane to grant them the strength of a balanced existence.
Secundus is a shadowland, full of dark forests, rolling hills and hundreds if not thousands of isolated enclaves of all kinds of horrors possibly dreamed into being by the night terrors of peons or by misguided cults.
Secundus is not something made solely of dreams to fade in the daylight hours however, it has probably been there ever since the beginning of Primus and now creates it’s own monsters to plague it’s own peoples.
Other theories try to explain the origins of Secundus, that it is the creation of a Primal Convergence and that there was once more than a single Exostance, or that it is some kind of cosmic waste storage when Primus becomes too full.
Nothing as yet has been proven to conclusion, though many who debate this issue seem convinced they have evidence proving themselves correct.
Dangerously close to Primus, Secundus often mirrors, albeit in a darkened and warped fashion, events that we see here near the Focus. This ‘nearness’ which has nothing to do with actual measurable feet and yards, and can create dangerous feedback which draws the Mittlenacht to Primus in localized convergences.
The Mittlenacht has often been blamed for events seemingly without explanation, strange villages full of inbred peons, or sudden floods of the undead, whether this is true or not remains to be seen, but what is certain, many heroes will encounter the mittlenacht in some form in their career, especially further they travel from the Throne of Glass.