The general explanation of Nightfall describes the Outland as a plane that exists semi-concurrently with the prime material plane, circling the Earth's dark side. Both planes move with and around each other, drawing energy from the sun. Nightfall is when the Outland moves to obscure the earth from the sun, blocking it. This should not occur naturally, but during times in which the two planes are close, it is possible for such a thing to be forced. Generations of humans and seers have found in their conflict with daemons and the other fell races that there is an organized effort to bring about Nightfall, exploiting just the right conditions to tip the scales in their favor. The desolation of humanity is in some respects a side effect - an Outland flush with sunlight will explode into the material plane and would in effect wipe out an unprepared humanity - the loss of sunlight would quickly dry up food supplies and the ensuing Outland mists would overwhelm the planet. There are a number of discoveries and conversations noted by scholars over the centuries, some daemons see humans as a nuisance best eliminated, others (as well as other Outland species such as and especially the vampires) have taken steps to collect some humans as chattel for labor and consumption, and others just want all the power for their own internal strife and couldn't care less about earth and humanity. Either way, there is a concerted effort and alliance among a number of powerful Outlander factions to bring Nightfall to fruition - centuries of planning and preparing all for this one opportunity.