Demi-Planes in the Ethereal
Large globs of proto-matter swirl about in the Ethereal, gathering other proto-matter, until a critical size is reached. At that time, the matter begins to pull away from the plane, growing its own Border Ethereal around it. This mass of proto-matter becomes a demi-plane.
Demi-planes are planes still in the formation process. They do not have a set place in the cosmography of the planes. They are not inner planes, though they exhibit some of those tendencies. Nor are they small Prime planes, although some are not hostile to Prime Material life. Some demi-planes fall in on themselves and break up, others merge with an inner plane or Prime Material plane. When they merge with other planes, they immediately become part of the larger planes and are subject to all the laws that govern those planes. Sages say that if a demi-plane grew large enough, it could envelope a large portion of the Prime, or separate to form its own universe. Rounding up evidence to support this supposition is hampered by a lack of permanence in the ethereal medium.
Each demi-plane operates on its own rules; gravity, vision, material make-up, and inhabitants all vary. Regular rules for spell effects may exist, but no one has figured them out as of this time. (Most spells have not yet been tried on these new planes.) The only guarantee is that each demi-plane is a finite area where rules break down.
There are four named demi-planes. Three of these have been visited in known history; the fourth may be only a legend.
The Demi-Plane of Shadow is the largest of these planes, and is constructed of equal parts of the Positive and Negative material planes. It is a murky realm filled with shadow monsters, some of which have earthly incarnations. lllusionists make use of this plane when they engage in the shadow walk spell. It is possible that natural ducts could be opened up in the shadows of the Prime Material plane. The curtain surrounding this demi-plane is silver.
The Demi-Plane of Time is almost as large as the demi-plane of Shadow, though its terrain has yet to be mapped. It is hostile to life, and its temporal manipulations extend into its Border Ethereal, such that some of those who return are hideously old or 1d10 years younger than when they entered (with the appropriate changes in appearance, knowledge, and abilities). The natives are inaccurately known as time elementals (a misnomer since they are not natives of the inner planes). They are described in Monster Manual II. The curtain surrounding the demi-plane of time is translucent with shadowy shapes visible beyond.
The Demi-Plane of Electro-Magnetism is also called the Electro-Magnetic plane. This plane is dying as its substance is slowly sucked into the quasi-elemental plane of Lightning. Most of the laws of the plane of Lightning are followed in this demi-plane. Scholars believe it is a matter of eons, even centuries, before it is completely absorbed. The only known natives of this plane are the Shockers. The curtain surrounding the demi-plane of Electro-Magnetism is copper colored.
The Demi-Plane of Imprisonment is only hinted at in the legends of those who are themselves legends. Legend says that a dark and chaotic force, evil beyond the pits of the Nine Hells, once dominated this region of the dimensions. The forebears of the beings known as the Great Powers of all alignments defeated this creature and its servants and sent them in huge hunks of matter spiraling into the Negative Material plane. But the destruction was not complete, and legend says that far beyond the realms traveled by sentient beings, there are large demi-planes still ruled by the irreparably evil creatures. The color of this plane’s curtain is unrecorded-travelers are strongly cautioned against investigating vaporous curtains that do not match known colors.