Cosmos Overview

The Million Spheres cosmology uses the Myriad Planes concept from the DMG, and Beyond Countless Doorways by Monte Cook as a basic framework.

The cosmos teems with a multitude of worlds or realms, most of which float in space like stars and planets in the sky.  The majority of these worlds are normal, although some can be considered quite fantastical.  Those worlds taken over by a god or pantheon are known as Dominions, or commonly referred to as heavens or hells.  The swirling worlds of predominately elemental matter and energy near the center of the cosmos are known as the Inner Planes, and are the homes of elemental beings and primordials. The emptiness of space and the darkness of the Void separate the worlds and realms.

Few planes are singular. There is not just one plane of shadow, but the shadows of many worlds. There is no plane of fire, but many worlds centered around fire, each unique. Every world is just one of many. A lot of worlds are similar to the character's home world.  Planar connections (conjunctions) become very important. Not every plane or world can be reached from every other plane.

The majority of campaign worlds are like our solar system, with vast amounts of airless space between the worlds and planets.  Each system is considered a "plane of existence" for purposes of magic and spells.  Area with high magic may resemble the description of Wildspace from the new Spelljamming book, while lower magic areas would more resemble our own solar system.  Many systems/planes would likely have a mix of areas.

Once you reach the "edge" of such an area/system you may see the faint shimmering glimmer of the astral boundary (curtain).  It is a this point you may transition from space/wildspace to the Astral, allowing for travel at higher speeds to other areas or systems.  You may also ignore the faint glimmer and continue onward into the darkness of the Void Between the Stars on a long and slow journey to other worlds/systems. (good luck on replenishing your air or avoiding death from old age) (and try to avoid the Great Old Ones).

The Ethereal Veil (or Spirit Realm) The Ethereal Veil is a gray shadow of the world. For the most part, the Veil functions exactly as described in canon.  Standing in the Veil, you see a gray shadow of reality. You can see the misty forms of buildings, of trees, of people going about their business… but you cannot be seen or heard, and you cannot affect the adjacent reality. With few exceptions, the Veil is empty. It reflects the adjacent reality, but it holds nothing of its own, and for this reason people rarely stay there for long; there’s no food, no water, and most of the time, no people. As noted earlier, the Veil is an extension of whatever plane you’re currently on.  Haunts and Borders are two special places where the Ethereal takes on more concrete form.

The Ethereal Veil extends from the plane its attached to, but no farther. There’s no Deep Ethereal, no curtains to other planes; the Astral Plane is the primary corridor for travel. However, the Borders are where planes come together, and it may be possible to move between material and the connected planes in such places.

Limbo is a plane of pure chaos, a roiling soup of impermanent matter and energy. Stone melts into water that freezes into metal, then turns into diamond that burns up into smoke that becomes snow, and on and on in an endless, unpredictable process of change. Fragments of more ordinary landscapes — bits of forest, meadow, ruined castles, and even burbling streams — drift through the disorder.

Limbo is both the underlying foundation of the cosmos, and the swirling mass of matter and energy that spawns worlds from its center.

The Astral Plane is a great starry sea, the same above and below, with swirling wisps of white and gray streaking among motes of light from appear as distant stars.  It is  the realm of thought and dream, where visitors travel as disembodied souls to reach other worlds.  Most of the Astral Sea is a vast, empty expanse. Visitors occasionally stumble upon chunks of rock drifting forever in the dim star lit emptiness. Much more commonplace are color curtains -  magical veils of light that flicker like an aurora borealis, marking the border of unknown worlds. Some of the worlds out there could be as fantastical as you like, say an enormous water/vapor planet that works as a floating ocean, or a ringworld built of enormous trees by ancient elves.