Within the universe of the Age of Arcana, there are the Planes of Existence, and the Realms of Reality. The Planes of Existence are different worlds, separate locations outside of the direct influence of cause and effect taking place within the others. The Planes are many and varied, but are all connected by the various shades of the Realms of the Dead and by the Astral Sea.
The Astral Sea
Connecting all planes, and physically parallel to the spiritual Realms of the Dead, is the Astral Sea. The Astral Sea reaches through-out all planes and all places, ebbing and flowing between each, and permitting travel through magical portals and occasionally even enchanted vessels. The Astral Sea is often mistaken as the origin of magic, but this is a simple misconception - The Astral Sea actually connects the ley-lines of the various planes in a metaphysical sense, bringing the force of creation to and away from the planes themselves, being a connecting force rather than an origin.
The Mortal Plane holds all prime material reality, from the farthest reaches of the outer void to the central sun. The realm of the dead that flows through the mortal plane is known as the Realm of Shadows. The mortal plane is the origin of most mortal life, from flora to fauna, and every human, dwarf, elf, halfling, orc, and goblin. This plane is considered central in the cosmology of reality, but only so far as it is central to our understanding and study of reality. The basis for all laws of physics and magic are set to the standards of the mortal plane.
The Celestial Planes are the home of the Celestials, creatures of Good brought into being during the very creation of the world. The realm of the dead that flows through these planes is known as the Sea of Hope. Few living mortals have ever experienced the celestial planes and returned, and many who have went mad in the experience. Cities of solid light, rivers of flowing silk, paths paved with the dreams of the young. No true cosmologist has taken any first hand mortal description of the planes to heart, and no celestial thus far has been willing to divulge the information.
The Infernal Planes are home to the Infernals, creatures of evil birthed during the creation of the world. The realm of the dead that flows through these planes is known as the Sea of Despair. Mortal accounts of the infernal planes are far easier to come by, and it seems as though the Infernals themselves can be almost amiable when it comes to interaction with the mortal plane. No doubt this is due to their incessant need to conquer and devour mortal souls. Regardless, the accounts speak of a land of misery and pain, with mountains of bone and rivers of boiling blood. Few who return from the planes remain as they were when they left for it, and it is believed by many researchers that the planes themselves hold a corrupting influence on the mind.
The Elemental Chaos is a plane of broiling madness, a land so utterly devoid of order and sense that to visit flirts with insanity. The primordials who did not cow to the dragons at the end of the Primordial War were banished from the material plane, and many who were too wounded to reach other realms or other worlds fell dead within the Astral Sea itself. As more primordials fell to their wounds, their bodies began to converge into a singular plane. The lands that rose up to form the plane were so utterly dichotomized that the resulting chaos shattered them, and the Elemental Chaos came to be. Lands of fire warred with lands of water, lands of earth warred with lands of air, and the shattered souls and spirits of the elementals broke into separate beings, becoming the elementals known in the present day. As time moved on, the separate lands of the Elemental Chaos spread further apart, and the planes of Fire, Water, Earth, and Air, all came into their own, with the central planar axis becoming more widely known as the Elemental Chaos itself. The outer planes of the Elemental Chaos are considered less hospitable to mortal life, and thus are visited far less often, being lands of infinite earth, infinite fire, and so on. The central axis is comparable to the mortal plane, had the mortal plane been so utterly without design and function.
Realms of the Dead
The Realms of the Dead are a single, vast ocean of energy that stretches out across the different planes of existence, carrying the souls of intelligent beings along its currents. The souls of the dead are, by default, carried off to the Well of Souls that recycles their essence to be reborn into a new body. Should a soul live life in accordance with the whims of the Celestial or Infernal forces, their soul is taken to the respective planes as their final resting place, instead of the Well of Souls.
Realm of Shadow
The Realm of Shadow flows through the Mortal Plane, carrying the souls of mortals off on their fated path. The Realm of Shadow is a twilight place, both of shadow and of dusk and dawn. One can find both positive and negative energies within the realm of shadow, as it is a realm of balance in all things - order and chaos, good and evil, light and dark. For mortals visiting the realm, it is a mirror of the mortal plane, filled with souls of the living and the dead. Within the Realm of Shadow may be found paths to the Feywild and the Shadowfell, both reflections of the material plane, the realm of shadow, and each other. The Feywild represents the neutral forces of life and growth, while the Shadowfell represents the neutral forces of death and entropy.
Sea of Hope
The Sea of Hope is a realm of positive energy. According to all known research, positive energy originates from this realm and flows from it to all other planes. Every spell, every effect, every living being, derives its positive energy from this realm. Few have been known to visit the Sea of Hope, as it is particularly difficult to do so with a physical body, but by all accounts it is a place of golden light and healing warmth. There is no suffering, no terror, no hatred or cruelty. The Sea of Hope flows through the Celestial Planes, and carries the souls of the favored dead to their final rest within.
Sea of Despair
The Sea of Despair is a realm of negative energy. According to all known research, negative energy originates from this realm and flows from it to all other planes. Every spell, every effect, every living being, derives its negative energy from this realm. Few have been known to visit the Sea of Despair, as it is particularly difficult to do so with a physical body, but by all accounts it is a place of darkness and misery. There is no healing, no pleasure, no love or kindness. The Sea of Despair flows through the Infernal Planes, and carries the souls of the favored dead to their final rest within.
The Feywild is one of two reflections of the Mortal Plane and the Realm of Shadow. The Feywild is the reflection of life and growth, a realm of energy and mania. Within the Feywild, one may find life of all sorts, from verdant forests to deep swamps and everything between. The fey as well as many races and species of the mortal plane trace their origin to the Feywild, and indeed most of those creatures still flourish within. The courts of the Fey are centralized within the Feywild, and many nature deities either trace their origins from the realm, or choose the realm for the location of their own courts. Those primordials of life and growth who did not meld with the Elemental Chaos may instead have chosen the Feywild for their refuge, and many live there to the present day.
The Shadowfell is one of two reflections of the Mortal Plane and the Realm of Shadow. The Shadowfell is the reflection of death and entropy, a realm of stillness and depression. The Shadowfell holds locations of darkness and solitude, with rot and cold silence being the norm. Little natural life grows within the Shadowfell, and most of it stems from what negative energy flows through the realm, rather than thriving off of positive energy like life within the material plane. Creatures of darkness and shadow lurk within the Shadowfell, and endless waves of undead can be met within the crags and upon the dry plains of the realm. The occasional deity associated with darkness, pain, murder, and the like, are known to call the realm home. Those primordials of death, decline, and other such mortally wretched things who did not meld with the Elemental Chaos chose instead to take refuge within the Shadowfell. Many of the primordials faded into the land itself, but many others can still be found within to the present day.