Fantasy games like DnD include the presence of alternate dimensions or "planes". The presence of these planes may be very important to your world, and should you decide to use them you should plan for how they work, how they affect the primary plane of existence, and how they can be reached.
Are there other dimensions, planes, or worlds?
The existence of other dimensions has heavy implications for your setting. The settings of Warhammer and Warhammer 40k both have an alternate dimension called "The Warp", which is the home of otherworldly horrors like demons. Dungeons and Dragons worlds largely share a singular cosmology, often called "The Great Wheel". Norse mythology depicts several worlds as parts of the "World Tree". The presence of these dimensions has far-reaching implications for the way the world works.
What are they?
Alternate dimensions can be many things. Some or all of the following examples might exist in your setting:
- Butterfly Effect: Every decision creates a branch in reality, and each world is defined by a different set of decisions made after one root point in history.
- Embodiment of a Concept: A different plane might be the physical embodiment of a concept, like "Good", "Fire", or "Hunger".
- Pocket Dimension: An alternate dimension might be a pocket dimension which exists within the real world, like the Matrix.
- Separate World: Other dimensions might just be separate worlds from the primary dimension which look and function in largely the same way.
- Shadow of Reality: Some dimensions might be a weird echo of the "real world" dimension. DnD's Ethereal Plane, Plane of Shadow, and Feywild are all good examples.
- Void: An alternate dimension might be a featureless void.
Where are they, and how does someone reach them?
"Where" is a difficult question to answer in many interpretations of extra dimensions, but in others it's a perfectly valid question. Other dimensions might be places which you can walk to, but which are very different from the normal world. Some planes mirror reality; in the Warhammer 40k setting, The Warp is an omnipresent alternate dimension which you can reach from anywhere, and which occasionally bleeds into the real world (especially around the Eye of Terror). In DnD the Etheral plane coexists with the material plane, and ethereal creatures can see into the material plane like looking through a fogged window. Other planes in DnD are far-off places which must be reached by powerful magic or by traversing a series of portals.
The means by which you reach other dimensions helps to inform how they interact with the primary dimension. In Greek mythology, you could reach Hell by descending certain caves in some mountain ranges, and you could reach Olympus by climbing mount Olympus. In the Halo universe, "slipspace" is reached by tearing a hole in spacetime with complicated device called a "Shaw-Fujikawa Translight Engine". In DnD, different dimensions can be reached with powerful magic, or by traversing portals or places where the barriers between adjoining dimensions are weakened and unsuspecting inhabitants can fall between the material plane, the shadow plane, and the feywild. In the Spelljammer setting, planes are contained in "crystal spheres" which can be traversed by powerful ships called Spelljammers. Assuming that these means of transportation are two-way, if your alternate dimensions are inhabited you may need to consider what happens when the inhabitants of those dimensions enter the primary dimension.
Examples of Planar Cosmology
- The Great Solution: The planes rest in a vaguely defined cylindar. At the top of the cylindar sits the positive energy plane (source of "good"), while the negative energy plane (source of "evil") rest at the bottom. Energy flows from the two, intermixing in equal parts like an acid and a base, each neutralizing the other overall, but never reaching perfect equilibrium. This mixture makes up the Astral sea. The material plane exists at the exact center where the mixture is the most even, while other planes form in the churning around the center. Planes closer to the top are more good-aligned, while planes closer to the bottom tend toward evl. Planes closer to the edges might wink in and out of existence, or shift in appearance and substance like the changing of the seasons as the makup of the astral sea fluctuates.
- The Great Wheel: The classic DnD cosmology, the material plane and its echoes rest at the middle of two concentric rings of planes. The inner ring features elemental planes, while the outer ring featurs planes closely tied to the 2-axis alignment system.
- Mount Olympus: Based on the Greek mythology, Olympus is so tall that its peak is actually another plane. Hades (Hell) is located somewhere deep underground, and somewhere far below that the titans are buried. Far to the west the Elysian fields serve as the resting place of great heroes.
- Myriad Planes: Imagine a ball pit where every ball is a different plane, intersecting with every ball it touches. Go jump into it.
- The Omniverse: A simplified version of the DnD cosmology, the Omniverse model features a material plane and its echos, an "elemental chaos", a heaven, and a hell.
- One World: All of the planes coexist in one dimension, and can be reached by normal (though often extremely difficult) means. C. S. Lewis's novels (The Lion, The Witch, and The Wardrobe, etc.) could be said to have a One World cosmology based on The Voyage of the Dawntreader, though the fictional setting of the novels is linked to the real world through portals.
- The Orrery: A variation of DnD's cosmology, the Orrery abandons the fixed position of the planes, and instead they float closer and further to the material plane like planets, exerting varying influence based on their distance.
- The Otherworld: The real world has a twin dimension where everything weird lives. Warhammer's "The Warp" is a good example of an Otherworld cosmology. Not all interpretations are so sinister as The Warp. The otherworld
- Solar Barge: The material plane is flat, and the opposite side of the world is a dark hellscape. The sun moves between the planes at various stages in its orbit around the material plane. The Solar Barge cosmology is based on Egyptian mythology.
- The Winding Road: Every plane is connected along one weird road, and by following the road you can reach every plane. The borders between the planes are kind of vaguely defined. They might be as subtle as a gradual lengthening of the grass, or it might be an actual physical barrier.
- The World Tree: The world tree originate from Norse mythology. Yggdrasil attaches the nine worlds, each of which is functionally another dimension or plane of existence.
Example 1 - Shadow of Olympus
The Olympus model is perfect for a greek-themed setting. Hades can be accessed through certain cave networks which lead to the river Styx, and Olympus is a ridiculously huge mountain at the center of the world with the seat of the gods at its summit.
I want to include elemental forces (possibly as the origins of the titans, who the greek gods don't particularly like), so let's include the elemental chaos as a flat ring around the edge of our world. Maybe the elemental chaos is a churning mass of chaos barely held back from the fraying edges of the material world. because I want to use this setting with something like DnD or Pathfinder, let's say that there's also an ethereal plane for ghosts to hang out in, and the Abyss hangs off the bottom of Hades, and contains awful things like demons and titans, locked away far from the world above.
Example 2 - Space Grease
Dieselpunk in space doesn't really need other dimensions. We might consider a "hyperspace" or "war space" concept for FTL travel, but I think it's fine for there to be some sort of real-space FTL travel mechanism, making other dimensions unnecessary.
Example 3 - Heroes of Tonesvale
Alternate realities are a classic plot element in comic books, but it's rare to have a consistent, defined set of alternate dimensions. There might be some dimensions which exist largely for the theme of specific characters (like the weird dimension where Doctor Strange apparently likes to hang out), but they can be created and discarded when specific characters need them, much as they are in the world of comic books.