Advanced Dungeons & Dragons Planescape Supplement by TSR / Wizards of the Coast
Beyond the Prime’s infinite crystal spheres, beyond the endless realms of the Outer Planes, lies the absolute reality of the Inner Planes.
The cornerstone planes of Air, Earth, Fire, and Water are the most well-know Inner Planes, but they’re only the beginning. From the deadly purity of the Positive and Negative Energy Planes to the unbearable filth of the Plane of Ooze to the eternal storm of the Plane of Lightning, the Elemental Planes are the substance of existence itself. These eighteen planes for the foundation of the entire multiverse: they are the cauldron from which all energy and matter are brewed.
The Inner Planes, by Monte Cook with William Connors, was published in November 1998.
The Inner Planes (2e) is available through DriveThruRPG in Print and PDF.
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Voices From The Pulpit 