It doesn't take much searching for any one person to feel tiny. Even simply taking in the sheer size of our solar system or even just Earth is enough to elicit such sentiments in people.
Now imagine looking out beyond the Milky Way or even the Local Group, staring out at what seems to be an infinite stretch of empty voids. Imagine there are entire other galaxies out there within the universe just as expansive as our own. Now reach beyond that, sailing beyond the bounds of a single universe.
This is a fine start to understanding the totality of existence, though only a start.
To us tiny mortal lifeforms, the incalculable number of worlds beyond comprising all of existence seems infinite. This absurd seems to go on far out into eternity looming over all things like an omen of death.
Despite the deep existential terror such an expanse is seemingly designed to instill in mortals, there are many who wish to tame it or learn from it.
These groups scour their local clusters of universes in search of the secrets the void and life far beyond the feeble preconceived barriers of space time. Many magical beings, especially Angels and demons, are able to move between the folds of reality with ease as their very essence is made to do so. Unfortunately for mortals these journeys are nearly impossible due to the great lengths needed for any of the many means to travel between universes within their lifetimes.
For better or worse, this has yet to stop several groups from such an undertaking.
Where things really get interesting, however, is the layers which form as universes come into and out of existence. These layers of reality form from the inner workings of physical universes and their contents and take any number of strange forms.