Prime Material (inner plane)
The Prime Material Plane includes the planet Earth and the vast universe that surrounds it, as a separate and distinct realm of existence. Within this plane, the familiar natural laws that are thought to apply everywhere exist. Here, the setting and mortal creatures that inhabit D&D play out the drama of ordinary existence, hampered by things that obey the laws of physics, reality and magic.
Yet the Prime Material is just one of many planes, perhaps an infinite number. These other planes, known collectively as the "Planescape Universe," encompass a staggering array of diverse and fantastical realms, each with its own unique characteristics, inhabitants, and rules that often defy the conventions of the Prime Material plane. These are linked together by a mystical concept within various mythologies and cosmologies as the "Tree of Life" also called the One Tree or Crann Bethadh.
The Planescape Universe
The Tree of Life is often depicted as a colossal, mythic tree that transcends the boundaries of reality. It is said to have roots that extend deep into the earth, branches that reach high into the heavens, and sometimes even additional roots or branches that connect it to other planes of existence. The tree's symbolism may include themes of growth, renewal, balance, and the cycle of life and death. The Tree of Life exists as a cosmic axis or a central pillar, the roots of which reach deep into the Prime Material Plane, including the Earth itself. The branches of the tree, which serve as pathways, reach up into the Astral Plane. Along these pathways, moving through the Astral Plane, creatures from the Prime Material can gain access to other planes of existence.
The Astral Plane is a fundamental plane of existence that predates the Tree of Life. Some have described it as an incomprehensible expanse without physicality, where everything has an ephemeral quality. Objects and creatures in the Astral Plane may appear as shimmering, translucent, or ghostly forms.
The Ethereal Plane
The Ethereal Plane is a separate plane of existence that exists alongside the Prime Material Plane (the physical world) and other planes. It is often described as a sort of "buffer" or intermediary realm that coexists with the material world. The Ethereal Plane is typically depicted as a misty and translucent realm, and it allows travelers to see into the Prime Material Plane while being partially detached from it.
Influence upon the Prime Material
Amidst this bewildering cosmic structure, where each "leaf" of the Tree of Life represents a distinct planar universe, dwell immensely powerful entities whose influence upon their surroundings far surpasses the capabilities of beings residing within the Prime Material Plane. These are the Gods, who unlike mortals bound to the limitations of their home realm, the Gods are omnipresent and omnipotent, capable of moving effortlessly across the tapestry of the multiverse.
Some of these, those who are interested, watch over the Prime Material Plane and its inhabitants with benevolence, indifference or capriciousness, depending on their individual natures and agendas. They bestow blessings upon their devoted followers and grant divine power to chosen holy persons, enabling them to perform miraculous feats through worship of the God's "religion." As immortals, the Gods can play havoc with the Prime Material — yet as they are free to oppose one another, the greater number of these beings continually seek to build a cosmic balance between themselves.
It's within this balance that the mortal creatures of Earth dwell and live, in large part ignorant of the underlying reality.
See Mythology
