The Demi-Lich, from a physiological level, has to work on the assumption that everything dies, even immortal liches over time.
but it does cause some cognitive dissonance because the astral plane is supposed to be ageless, and if that's the case, the lich should be well preserved as if in a psychic Tupperware container (especially in the tomb on annihilation).
I guess the way I'd explain it from an in story perspective is that the demi-lich is kinda a "plan b" where, like Koschei, they store their phylactery somewhere else but the gods/high level party, instead of destroying all forces and remains, curse it reducing it's physical properties as sort of a unliving hell (maybe because it cheats by hiding all it's phylacteries in a gygaxian manner and the pcs get outside help to contain this monster).
The Deml-Lich: Ages past, a human magic-user/cleric of surpassing evil took the steps necessaly to preserve his life force beyond the centuries he had already lived, and this creature became the lich, Acererak. [...] Eventually even the undead life force of Acererak began to wane, so for the next 8 decades, the lich’s servants labored to create the Tomb of Horrors. Then Acererak destroyed all of his slaves and servitors, magically hid the entrance to his halls, and went to his final haunt, while his soul roamed strange planes unknown to even the wisest of sages. [...] All that now remains of Acererak are the dust of his bones and his skull resting in the far recesses of the vault.
A supremely evil human magic-user or cleric can exist far beyond the natural span of life by using certain arcane secrets. This creature, the lich, can exist for centuries.
Ultimately, its life force eventually wanes. The lich form decays and the evil soul roams strange planes unknown even to the wisest of sages. The remaining force is a demi - lich. "Demilich" is a misleading term, in that the hearer might believe that the creature is reduced in power. The term applies to the physical state of the lich. Only a small amount of the former lich 's body endures - dust, the skull, and possibly a few bones.
I don't think the demi-lich is actually anywhere in the adventure, but there's a monster listing. Perhaps it's just included as a compilation of new S-series monsters?
In any case, this is Acererak writ generic. Not even liches live forever, demi-liches are liches whose bodies have decayed.
Demilich is a misleading term in that one might assume the "demi" refers to status. However, it refers to the state of the lich. Only a small part of the body of a lich remains - dust, the skull, and possibly a few bones.
A lich (q.v.) is a human magic-user and/or cleric of surpassing evil who has taken the steps necessary to preserve its life force after death. Ultimately, even the undead life force of a lich begins to wane. Over centuries the lich form decays, and the evil soul roams strange planes unknown to even the wisest of sages. This remaining soul is a demilich.
The demilich is not, as the name implies, a weaker form of the lich. Rather, it is the stage into which a lich will eventually evolve as the power which has sustained its physical form gradually begins to fail. In most cases, all that remains of a demilich's body are a skull, some bones, and a pile of dust.
When it has learned all that it feels it can in its undead life, the lich will continue its quest for power in strange planes unknown to even the wisest of sages. Since it has no use for its physical body at this point, the lich leaves it to decay as it should have done centuries ago.
Wizard and priest drow may become demiliches in the usual manner. [...] Drider liches have never been known to make the ascension to demilich. Whether this is because no one has yet discovered such a creature and lived to tell about it or because Lolth enjoys allowing the things to quest for the unattainable is unknown.
Particularly powerful liches sometimes learn the secret of fashioning soul gems, and so evolve to demilichdom. Demiliches are free to roam distant planes in astral form, leaving behind their physical remains to brood in dark tombs of horror.
The immortality granted to a lich lasts only as long as it feeds mortal souls to its phylactery. If it falters or fails in that task, its bones turn to dust until only its skull remains.
Few liches seek to become demiliches, for it means an end to the existence they hoped to preserve by becoming undead. However, time can erode the lich's reason and memory, causing it to retreat into its ancient tomb and forget to feed on souls.
So now it's a thing that happens when a Lich starves to death. There's also a sidebar on Acererak-style liches, where he planned for it and the soul-stealing gems in the skull are there to feed his phylactery.
So, in summary:
TSR Demiliches are just what really old liches look like. "Really old" is in centuries, incidentally.
3E Liches are the Lich's Lich, where a Lich takes their soul from their phylactery and puts it in a soul gem.
4E may or may not have had demiliches, but Wikipedia didn't say anything and a quick google didn't help.
5E Liches are what happens if a Lich starves to death.
4e's Open Grave has stats for Acererak (26th level solo controller), and for Acererak Constructs (13th level solo artillery). Acererak created the construct as a decoy in the Tomb of Horrors; the real Acererak is in the Fortress of Conclusion in the Shadowfell. There's also a Lich Vestige (26th level minion) in the MM, intended as a destroyed lich that didn't have a phylactery.
Demilich as a hazard, maybe in a skill challenge combo, would be an interesting change from the standard combat scenario.
On undead as hazards: GURPS Dungeon Fantasy has Horrid Skulls, which are evil spirits that inhabit skulls and project a damaging aura. Being within the aura means taking a low amount of automatic damage every turn. The skull is not animate and is rather easy to destroy, but the monster description notes that the "correct" way to use it is to place one amid a giant pile of normal skulls, or hidden in a secret alcove in a wall.
Well, if you look at the actual history, GG designed the Tomb and Acerak for a specific purpose: his gaming group had gotten too big for its britches, and he wanted to take them down a notch. He succeeded. So, he made a campaign that was wickedly fun for a DM for a change. In fact, the whole S series is for DMs and not players. I really don't think he meant for Demi-liches to be prolific. I honestly believe Acerak should have been the only one.
Me too, though I also like the archivist demilich from the Epic Level Handbook. In a setting where she appeared, though, I would make her the only one.
Making specific monsters generic is kind of a recurring D&D thing. You most often see it when they take a monster from a given setting an remove any setting-specific fluff from it, but there's also the "taking a singular villain and generalizing it into a whole species" thing that went on with Acererak. To a lesser extent it kinda happened to Strahd as well. Ravenloft went from a single adventure to a whole setting, and "Dark Lord" became a category.