First Mention: To Thine Own Self...: Chapter Seven (November 20, 2006)
The "mortal realms" appears to be a generalized or "catch-all" moniker used by extradimensional entities to refer to the portion of the Cosmos that forms the primary habitat for mortal, non-entity beings such as humans. The mortal realms comprise of individual but linked universes bound together by threads of probability and possibilities, and are thus bound by the four perceivable dimensions (length, width, depth, and time). Extradimensional entities have the ability to enter and manipulate mortal space seemingly at will, although strict regulations prohibit modifications beyond simple repairs to time and space anomalies.
According to an account given by Mischief and relayed to Todd, most entities care nothing about the mortal realms, living out their existences and seeing mortals as nothing but "insignificant specks in the unpleasant but necessary plumbing of the Cosmos." However, the Gamester taught her that the entities existed for the benefit of mortals and not the other way around. "The extra-dimensional realms are a support structure to keep the mortal metaverse intact. Mortals are what give the Cosmos purpose to exist. What that purpose is is debatable, but it's clear that [mortals'] tiny lives, at least in summation, are why [extradimensional entities] exist." Most entities, however, tend to forget this fact, viewing mortals lesser beings barely worth acknowledging. There are a few entities, though, like the Gamester and Justice, who realize just how precious mortality is and have pledged themselves to protect it. "What's more frightening", Mischief concludes, "are those who see mortals' importance... and seek to destroy it."