And now I can finally answer the question I mentioned being asked on Friday.
Part 5: The Destruction of the Nightspire
It should come as no surprise to anyone that the Beholders rebelled against their Illithid masters, and a bloody, genocidal war was fought between them. What should come as a surprise is how it ended.
The Illithid race had taken residence within the Underworld's version of the Dayspire (hereafter called the Nightspire to avoid confusion). The Beholders, seeking to topple their masters from their place of power, decided to destroy that fortress and cast them out into the light, which the Illithids shunned.
It's debatable which is more impressive: the fact that the Beholders actually managed to destroy a monolith of infinite height and immense width, or their startling lack of thought regarding what would happen as a result of that destruction.
For one, a spire of infinite height, when toppled, must perforce fall forever. To this day, mountain-sized chunks of Nightspire strike the surface of the Underworld regularly in an infinitely large spread.
Second, by virtue of having one side constantly exposed to the Positive Energy and the opposite to Negative Energy, the Nightspire (like its cousin, the Dayspire) was an immense magical battery, which is why the Illithids took residence there in the first place. When these charged fragments of Nightspire strike the surface, they explode in a cataclysmic shower of energies. Sometimes this is Positive Energy, and sometimes it's Negative. Sometimes everything is turned to glass for a mile. Other times, life is spontaneously created. Sometimes that life is a fluffy bunny. Sometimes it's an owlbear. Sometimes it's a Tarrasque.
Third, without the Nightspire to counterbalance the topside's Dayspire, the entire disk of Pellatarrum sank until it found a new equilibrium. This resulted in the legendary "Day of Rising" on the topside, where the Positive and Negative Planes appeared to rise from midway across the disk to their current 10:00 am positions. The topside has been the better for this ever since... the underside, not so much, as both "suns" have apparently sunk below its horizon.
Fourth, recall that in order to practice magic, a caster needs 8 hours of rest to regain the ability to cast spells. With the destruction of the Nightspire, the entire underside lost its day/night cycle. Without that cycle, there are no circadian rhythms; without those rhythms, there is no rest, and therefore no ability to practice magic. In a very real way, the Spire itself was a regulatory mechanism for magic, and without its governor, magic went berserk. In a world without observable time, "instant" spells can last forever and "permanent" spells can wear off within minutes.
In short, the underworld is now a hellish, blasted landscape of eternal explosions, erratic magic, and random bursts of creation and/or destruction. No sane creature would want to live there. This of course makes it the perfect destination for someone seeking to create or destroy an artifact, or risk death for a shortcut to power. Perhaps like Siegfried, the character inherits a permanent Stoneskin effect, or learns the secret to casting a level two Fireball by studying the magical environment.
Or perhaps he's devoured by a herd of vorpal bunnies.
Regardless, nearly all of the underside's original inhabitants retreated into deep caves in order to survive the apocalypse. Eventually, some of them were able to travel through the underdark into the caves on the top side of the disk. This is why nearly all aberrations are found in caves -- they are slowly migrating upwards.
The Fine Print
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