First, though, a correction: I had said that the etheric body dissipates shortly after death. Further research reveals this is only true when the etheric body is separated from the physical body at the time of death. If both bodies are in close proximity, then it seems clear that the etheric body lasts much longer—it slowly sublimates as the physical body decays.
It’s this slow decay of the etheric body that makes possible the creation of corporeal undead. Recall that the etheric body is imbued with vital energy—well, morbid quickening (i.e. “undeading”) occurs when the life energy of the etheric body is replaced by the energies of unlife. And where do these opposed energies come from? The two energy planes.
Positive Energy Plane
The etheric echo of the moment of creation (what some wags call the "cosmic orgasm"), the positive energy plane is an explosion of raw energy that pushes the elemental forces to codense and combine into mundane matter, and vitalizes all living things. It’s said to be found at the center of the universe, though it may be that travelling to it requires travel backwards in time more than space. A character exposed unprotected to the energies of the plane will gain 1d6x10 hit points a round, their every cell exploding with life energy. When a character’s hit points reach double their original total, he explodes--the matter of his being propagating out in space and time to perhaps give rise to whole biospheres. A character saved from such a fate will lose the increased hit points at a rate of 2d10 a round, but has a 30% of being changed in some way indicative of evolution to a higher state (an ability score increase, developing psionics, a innate magical ability etc.).Negative Energy Plane
The etheric shadow of the absolute end of all things, the negative energy plane is cold and dark, and the only motion is the inexorable fall into the final abyss. Negative energy manifests as entropy in the physical plane, but in the etheric realm it’s the energy of anti-life, propagating backwards in time, foreshadowing the death of the universe. Some philsophers believe the energy planes occupy the same space, but at opposite ends of time. Unprotected exposure to the negative plane results in the loss of 2d6 hit points and 1 level or hit die a minute, until the being shrivels and dies, becoming a mindless undead, unable to move in a place where time has lost meaning, doomed to circle downward for a subjective eternity into oblivion. A character saved before the end will not recover lost levels without the aid of magic, and may forever be haunted by what they have experienced—though there is a 30% chance that undead will now ignore him or her unless the character attacks them first.