The Positive and Negative Energy Planes (PWH)

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If the Elemental Planes are the cornerstones of all that makes up the physical universe, then it's the Positive and Negative Energy Planes that breathe life into (or take it away from) them. These are the planes of life and death. No truer opposites exist in all the multiverse.
These planes aren't the basis of energy in the sense of light or heat—that's the purview of the planes of Fire, Radiance, Lightning, and so on. The Positive and Negative Energy Planes are the planes of life force. All living things draw their energy from the Positive plane, while creatures of undeath draw from the Negative. Likewise, spells, magical items, and similar things based on promoting or taking life tie directly to these two planes.
The Positive Energy Plane is still a dangerous, terrible place to visit, however, despite its life-promoting nature. Its energy is simply too strong. Visitors find this plane even more brilliant than the plane of Radiance, with nothing but powerful white light. Its energies seep into bashers entering the plane until their bodies can no longer contain the energy, literally burning the poor sods out.
However, if a traveler is injured, the energies heal the wounds—and quickly. If a planewalker can work out some sort of system to transport wounded folks to the Positive Energy Plane for just a few moments and then retrieve them, she'll have created an infallible healing system. 
On the other hand, there's nothing useful to a planewalker about the Negative Energy Plane. Further, there's really no reason a basher'd ever want to come here. This place's got a lot of names—the Black Barrier, the Great Void, the Cold Land, or just Death. This plane steals energy and destroys the spirit. It is a black, formless plane of all-consuming hunger, feeding upon life energy. Addle-coves who come here without magical protection not only die, but their spirits are consumed forever, irrevocably—they're never coming back.
Some sodding idiots have actually made it possible to visit this plane. The Dustmen know of secret portals to a citadel that they maintain here, called the Fortress of the Soul. Great expense and magic has gone into making this place livable.
Surprisingly, there's life, or at least inhabitants, on both of the Energy Planes. The xeg-ya is a creature of positive energy, and the xeg-yi one of negative. Further, undead such as spectres, wights, and wraiths (and who knows what else) hover forever on the brink of the Negative plane, waiting to be loosed upon other planes where they can drain the energy of the Jiving.
Tarsheva Recommends: Just avoid these planes, berk.