Most adventurers travel the length and breadth of their land, facing an array of challenges from a variety of sources, such as monsters, dastardly villains, corrupt organizations, and even the perils of the landscape itself. These challenges, though daunting and difficult at times, are nonetheless rooted in the reality of the land around them. Magic can twist and distort to add an extra dimension to the challenges, but the scope of such power is necessarily limited by the hands that wield it.
These are all but a tiny fraction of the whole picture, however. They occur on the Material Plane, a sort of conjunction of extraplanar powers governed by invisible forces beyond the vision or even understanding of most people in the Material Plane. Beyond the boundaries of the Material Plane, what most accept as "reality," the wider multiverse opens with an infinite array of options that stretches the boundaries of what's possible – or breaks it entirely.

What is a Plane?

A plane is broadly defined as any reality separated by metaphysical borders. It could be infinite or finite in size, but each plane is a self-contained dimension with its own laws governing existence. Though exceptions exist, planes cannot usually be accessed by physical means. Most of the known planes of existence are not inherently hostile to life, though again, exceptions exist.

Definitions

Demiplane

A pocket dimension of finite size created from the raw protomatter of the Ethereal Plane. Certain spells can create demiplanes, usually as temporary locations, and some magical items access demiplanes as part of their enchantment. Many demiplanes can be accessed directly through a portion of the
Ethereal Plane known as the Deep Ethereal, but others are locked except through specific portals or gates.

Echo Plane

A reality "adjacent” to the Material Plane that includes some parallel aspects of the Material Plane, but usually not all. Each of these planes are an “echo” of the Material Plane that most planar sages believe are under the effect of a powerful exterior force. The Plane of Shadow is connected to the Negative Energy Plane, while the Plane of Faerie is connected to the Positive Energy Plane. The other known echo plane is the Plane of Dreams, which contains a distorted version of the Material Plane affected by the dreaming minds of sentient creatures. The Ethereal Plane consists of two regions, the Border and Deep, and the Border Ethereal can be considered an echo plane as well.

Energy Plane

A plane that consists wholly or partially of pure energy. The two known Energy Planes are the Positive and Negative Energy Planes, which contain the source of radiant and necrotic energy, respectively. These planes are hostile to life, though accessible pockets exist. The Energy Planes touch the Inner Planes and Material Planes, creating regions of mixed aspects that generally can be visited.

Inner Plane

A plane that contains the source of one of the four basic elements of existence – air, earth, fire, or water. These are referred to as the Inner Planes because they form the nucleus of the multiverse itself and, on a metaphysical level, are believed to be the core of all known reality. Each element is held within its own infinite plane, but the Inner Planes also “bleed” together, creating border elemental planes with non-opposing elements. Where air and fire meet, the Plane of Ash exists; where air and water meet, the Plane of Ice exists, and so on.

Great Wheel

The most commonly understood grouping of the planes understood by planar sages. Wheels, spheres, and circles are a common theme found amongst the planes of existence, so planar sages have connected the greater multiverse to the idea of the Great Wheel. At the center of the Great Wheel is the Material Plane, surrounded by the Inner Planes, and then orbiting around them are the Outer Planes. This is just one theory of the multiverse, however, and others exist that may be just as valid.

Layer

A subset of a plane of existence defined by different physical or metaphysical traits. Many of the known planes contain multiple layers, which function largely as separate planes themselves but are connected to the broader plane by easier accessed portals and gates.

Material Plane

The plane of existence that sits at the conjunction of all the other planes. Also known as the Prime Material Plane, each Material Plane is comprised of elements from all the other planes in varying degrees and amounts. They are infinite in size and scope and as varied as the rest of the multiverse, but they form the core of what most inhabitants call “reality.”

Multiverse

A generic term used to describe all of the known planes of existence. There are multiple theories of how the multiverse is connected together; the most common is the Great Wheel cosmology, but others exist as well.

Outer Plane

A plane of existence defined by a broad philosophical force. Most planar sages identify sixteen Outer Planes as part of the Great Wheel that correspond roughly to the intersections of good/ evil and law/chaos. An Outer Plane, in this sense, contains strong philosophical influence along these spectrums, which can include its absence as well. Mechanus, for example, is the plane of ultimate law devoid of good or evil, while the Abyss is the plane of ultimate evil and chaos. Some Outer Planes, especially the ones at the extreme, help those that align with it philosophically, but rarely are they directly hostile to non-aligned travelers.

Planar Sages

A sage that specializes in planar knowledge. In the context of this book, the planar sages include Astromarchus the Sage, Emirikol the Chaotic, Issilda the Unbreakable, LIllandri the Moon Mage, and Malakara the Warden.

Transitive Plane

A plane that exists primarily as a means to an end. There are two known Transitive Planes – the Astral Plane and Ethereal Plane. The Astral Plane is a strange realm where time functions differently and serves as the connective “tissue” between all of the other planes of existence. The Ethereal Plane is considered a Transitive Plane as well, though it only directly connects to the Inner and Material Planes.

Traveling the Planes

Each plane exists as its own self-contained reality, with separate laws governing how time, magic, and energy interact. As part of the larger multiverse, though, each plane is connected through a convoluted series of invisible pathways that crisscross the dimensions in varied and surprising ways.

Portal

Portals are the most common way to access other planes of existence. Simply put, a portal is an invisible locked doorway that directly connects two stationary points on different planes. To pass through a portal, a traveler needs to have the correct key. Certain spells known to planar sages allow for the close examination of a portal in order to ascertain the nature of the key, which could be anything – an object, a phrase, a motion, or anything else.

The nature of the key is usually related to the destination of the portal. For example, a portal to the Plane of Air may be a feather, while a portal to Acheron may be a small iron cube. A portal to Nishrek, the home of the Horde of Gruumsh on Acheron, maybe an iron cube inscribed with the Orc word for “war.” Portals typically remain open for 1 minute before closing, though certain conditions and portals may extend or shorten this.

Once opened, most portals show the destination point as clearly as though a viewer were on the same plane, so it can be easier to ascertain the nature of the destination once the portal is opened. This isn't a universal truth, however; some portals are opaque, revealing nothing about their destination, while others are simply swirls of vapor, perhaps colored in the same way as a color pool (see below). Most portals are the size of a typical door, requiring larger creatures to squeeze, but other sizes also exist. And some stretch to fit the size of the creature passing through it!

Typically, portals that lead from one layer of a specific plane to another layer are easier to open, and most function as simple gates.

Gate

A gate is very similar to a portal with one primary difference – using a gate doesn’t require a key. Some gates are open constantly, creating a permanent link between two planes, while others are based on time, opening or closing, based on seasons, the phase of a moon, or other external factors.

On a plane, access between layers is usually through open gates. For example, the Abyss and its infinite number of layers are all theoretically accessible through the Plain of Infinite Portals, the first layer. Gates there appear as vast yawning pits with no clear way of determining which one leads to where.

Like portals, open gates typically appear as windows with full view to their destinations, though this does vary. The size of a gate also varies more than a portal, and the typical gate is large enough for a Huge creature to go through without having to squeeze.

Two special types of gates exist in the transitive planes that make them unique. The Astral Plane consists of an infinite number of color pools, which function just like gates but are color-coded to their destination. For example, all color pools to Mount Celestia are gold, while color pools leading to Carceri are olive colored. The exact destination is obscured but fixed, so finding a specific color pool is as hard to find as anything else on the Astral Plane.

The Ethereal Plane holds a special type of gate called an ethereal curtain that separates its two broad regions, known as the Border Ethereal and the Deep Ethereal. The Border Ethereal functions similarly as an Echo Plane and overlaps the Material and Inner Planes, while the Deep Ethereal is a soupy mix of protomatter where demiplanes are born. Ethereal curtains separate the two. A creature moving from the Border Ethereal to the Deep Ethereal passes through the curtain without noticing it, but from the other side, the curtains are colored based on their destinations. They do not require keys and are always available

Vortex

A vortex is a special type of gate that leads to one of the Inner Planes. They usually appear naturally as part of a particularly powerful or supercharged weather event, such as a storm, earthquake, tornado, or typhoon. They typically last only a minute or so at the apex of the event, but the elemental denizens on the other side have an innate sense of their appearance and often swarm through them to spread their influence across as many planes as possible.

Conduit

The Astral Plane is considered by many to be the “backstage” of the entire multiverse. One of the reasons for this thought are the conduits – nearly invisible tubes that connect two points in the multiverse, like a gate or portal, but do so through a long channel that uses the Astral Plane as a “bridge.” Accessing a conduit may require a key or not and they stretch to fit any size creature passing through it. The journey is not as instantaneous as a gate or portal, and traveling through some conduits may take minutes or even hours.

On the Astral Plane, conduits can be seen but usually only faintly and at a distance. They twist in the silver void, largely invisible and mostly indestructible. Some planar sages believe conduits exist to transmit the souls of the dead to their final resting place and that mortal creatures can use them is an accident.

Magical Transportation

Powerful magical spells can be used to transport creatures across the planes as well. Most of these are the purview of elder archmages and learned spellcasters, but for the right price, a party of adventurers may be able to convince the spellcaster to weave the necessary spell.

Astral Projection allows for instant access to the Astral Plane, which functions as the “hub” of the multiverse in many ways. Plane shift can also take a party to a specific named location, and Gate is the more powerful variant. The etherealness spell takes the party directly to the Ethereal Plane, which can be useful in accessing the Inner Planes or the Deep Ethereal with its multitude of demiplanes.

Mapping the Planes

Maps of the planes tend to be unreliable at best. Some aspect of the infinite nature of the multiverse twists space in constant and unusual ways. Certain smaller regions can have reliable maps, especially on planes with a strong lawful influence, but largely, most planar sages don't bother with them. An entire organization called the Planar Cartographic Society exists to dispute this fact, though their claims of authenticity ring a bit false in more places than not.

Great Planar Paths

Gates and portals are great ways to navigate the planes, but finding and accessing them can be tricky or unreliable at times. There are three “great planar paths” that run like an undercurrent through the multiverse, making travel in specific regions much easier. They are the River Oceanus, which links several of the Upper Planes; the River Styx, linking most of the Lower Planes; and Yggdrasil the World Tree, a vast planar tree on Ysgard with roots and branches that extend across the planes.
A fourth planar path, known as the Infinite Staircase, exists as well, but its properties are not well documented by the planar sages.

River Oceanus

A sparkling river of deep blue water, the River Oceanus runs a revitalizing, gentle course through several of the Upper Planes. It's meandering course both starts and ends on the vast sparkling ocean of Thalasia, the fourth layer of Elysium. From there, it winds up the layers of Elysium, crossing into Krigala on the Beastlands and winding through Arvandor and Aquallor on Arborea.

The path of the River Oceanus is not as straightforward as one might expect. There’s no direct current directing its flow from one plane to another – it curves, twists, and doubles back on itself an infinite number of times. Passing between the planes and layers generally requires sailing down the river for 1d6 x 12 hours and making a DC 10 Wisdom check; users with proficiency in navigator’s tools can add their proficiency modifier to the roll. A success puts the sailing vessel in the desired plane, while a failure results in no change of plane.

No river routes exist that jump layers, and transport from one plane to another requires sailing from the top layer of the plane. For example, a boat sailing on the River Oceanus on Thalasia can only move to Belierin, and from there to Eronia, and then to Amoria; once on Amoria, it can sail to Krigala on the Beastlands or Arvandor on Arborea.

River Styx

The River Styx is a dark, malevolent watercourse that winds through the Lower Planes. Its waters are oily and wine red, churning with dark bubbles and filled with unidentifiable refuse and flotsam. The waters are dangerous, capable of robbing creatures of their memories temporarily or permanently, though fiends and the native Styxian monsters are immune to this effect. In the planes it flows through the River Styx creates a flash point of activity, especially in places like Carceri which make travel out difficult for those sentenced to the plane.

The River Styx flows through the top layers of Acheron, the Nine Hells, Gehenna, Hades, Carceri, the Abyss, and Pandemonium. It’s not a straight line, however, and it winds up and down through the planes in a confusing, meandering path. Traveling along the River Styx is more difficult than the River Oceanus.

A special type of yugoloth called a merrenoloth is renowned for sailing the River Styx. These cruel and greedy fiends are willing to take on passengers for the right price.

Yggdrasil, the World Tree

The plane of Ysgard is a realm of heroic proportions, where everything takes on a bigger and grander scale. The best example of this is Yggdrasil, the World Tree, an enormous tree that sits on one of the plane's floating earthbergs. It's branches extend into the fabric of Ysgard, allowing quick transportation between the earthbergs. Similarly, its roots stretch out to encompass the lower Ysgardian realms of Muspelheim and Nidavellir – and beyond. Yggdrasil’s roots penetrate the weave of the multiverse, creating a maze-like system of root tunnels allowing access to many planes.

Yggdrasil’s roots path link up to existing trees in mysterious ways. The older the tree, the better the chance of it holding a secret entrance into Yggdrasil’s roots. The best known planes with World Tree connections include Arborea, the Plane of Faerie, the Beastlands, Bytopia, Hades, and the Material Plane, but any place with an ancient forest could have a connection to Yggdrasil.

Accessing Yggdrasil's roots requires a key, similar to a portal, though the key is always related to some aspect of the entrance tree. The entrance usually sits in a hollow part of the tree or possibly below ground in the roots, and it smells distinctly like sticky sap. Inside, the root appears as a tunnel made of gnarled, twisted wood, that leads into darkness. There’s no sense of time in the maze, and branching tunnels constantly break off, leading to other places.