Lucy's Choice: Let's make Lucifer Parallels!

[Flyndaran]

It doesn't have to be a worst case Yellowstone scenario during the Cuban Missile crisis to induce a Lucifer. Just big enough to make Kruschev think it's a good time to launch a preemptive strike. Being wrong or even slightly off may allow the U.S. to launch some counter attack, and/or third parties to take the distraction as cause to start their own wars.
Realistic crap-sack global conflict but lacking united sides like in the world wars.

[PTTG]

Double is in the terminal stages of an ominous invasion.

Some kind of invasive humanoid is populating this world. They are initially a largely undifferentiated human-shaped mass grown in a pod; these forms are almost never consciously perceived by humans, however, as they themselves can become "cognitively invisible," and ignored by humans in normal conditions. Upon selecting a human host, the walker incapacitates them, then adopts the victims' memories and identity. They may generate a psionic "mask" to appear as their victim, but they also have the ability to produce fingerprints, hair, blood, and other material, and also reshape themselves, to some degree, to better match their host's proportions.

The pods themselves are produced asexually by the otherwise sterile invaders; they must take great care to hide these pods, as they must grow undisturbed for 16 months with a ready supply of water and food.

After assimilating their victim, they then go about simulating the victim's life, waiting for an opportunity to produce additional pods. When no humans are around and there are no immediate opportunities to produce pods, the beings prepare their surroundings and then enter into a torpor. When an entire settlement has been taken over, the dopplegangers start acting extremely isolationist, and convert any structures into pod shelters. Soon the entire community becomes a densely-populated nest of "invisible" meat creatures, which immediately start drifting outward into the surrounds.

The beings, in all stages, consume anything organic, and in fact have easy control of any domesticated animals. Indeed, they usually consume a victim's pets, leading to savvy humans being highly suspicious of anyone who has abruptly lost a pet.

This invasion proceeded secretly only for so long. Eventually it broke down. Now, enclaves of humans do battle with a horrifying, body-snatching, and incredibly insidious foe. And the number one fear of anyone is a stranger offering to help.

[PTTG]

The "invisible" beings can be discovered via technological instruments -- even something as crude as physical traps, as the meat creatures do not have human intelligence until they have captured a host/victim.

The dopplegangers are more difficult. Frankly they are a civilization-ending threat, in the long run. One of the simpler methods for detecting them is to enter an altered mental state, which dampens the psionic 'masking' effect, revealing the uncanny physical signs; most notably, the eyes lack sclera, and the skin is completely unblemished. But of course, one is now drunk, high, or maybe just completely terrified almost out of one's wits, and must be entrusted with life-or-death judgements of others.

I'd put this apocalypse no later than the early 2000s, as much later and computer neural networks could actually scan photos effectively.

In re-reading this, I think I'd replace the meat creatures with tiny wormlike parasites that don't have the psionic invisibility. Frankly, dropping the invisibility to gain an imperfect visible mask seems disadvantageous for something that simply wants to annihilate all human life and then just exist.

[PTTG]

Magic-based ones. Most of these are both high mana and have frequent mages, which seems uncommon until you realize that maybe these worlds like are naturally prone to fascinating magical disasters.

Igneous is a very-high mana world now ruled by animate mana-flare. It is the magical equivalent of an oil-well fire, with the addition of a fierce cunning. Only magically dead areas can host material objects for long.

Morph suffered a morphic breakdown. Humans are randomly transfigured into something else with virtually every substantial action. Simply punching someone is enough to scramble their form, as is awakening in the morning, sneezing, or simply walking through a door. The forms are usually mammalian, almost always animals, but nonetheless it's faintly possible to (for instance,) turn into a lit candle and then immediately get blown out by the breeze. Society is unstable as a result, and population is declining (hope you and your spouse happen to turn back into humanoid forms at the same time). Non-human objects and animals aren't effected. The effect tends to linger for a day or two after departing this worldline....

In Shadow, any place that is sufficiently dark serves as a gateway to a realm of evil spirits. A campfire can keep them at bay, and even a torch can burn them, but if the fire burns down to embers you'll be the next thing snuffed out...

And finally, Chronos has accelerated time for the growth of plants, the decay of materials, and weathering, as well as some other things. About ten years of time passes in 24 hours, meaning a tree can sprout from seeds and bear fruit between dinner and breakfast. So long as something is held in hand, this time acceleration is slowed to be no faster than the human clock, at least allowing people to eat a meal before it rots, but materials and equipment are extremely unreliable even if put in a backpack. Humans here live in the wild.

[Flyndaran]

Vaguely similar to Shadow is Darkness:
No natural light exists. The world is in perpetual darkness due to a mass global spell. If anyone magically or technologically produces light, not immediately, but eventually, it will create portals to demonic dimensions of unimaginable horror.

It's been only a few centuries since the spell, so only short lived animals have adapted fully. But the only humans left rely on innate magic senses while vilifying anyone using more advanced magic.

[PTTG]

Interesting. How long? Can someone light a candle to read a letter and then snuff it out?

[Flyndaran]

I'm in the midst of a very nasty cold and loads of cough medicine. So I'm really just letting my mind wander.
I wanted it to be unpredictable enough for world jumpers to be lulled into a false sense of security. If monsters jumped out immediately, it wouldn't quite be as mysterious or terrifying to those not already in the know.

[PTTG]

For reasons unknown, the star Vega emitted a colossal and poorly-understood flare in 1972. The light from the flare reached Earth in 1997, revealing the star to have been transformed into some kind of white dwarf. More urgently, it revealed that a dense plasma wave was accelerated towards the solar system at more than 0.8c. The wave conformed to no existing cosmological theories, especially considering that it remained coherent despite consisting of highly charged plasma.

It is expected to pass through the solar system, and affect Earth, in 2002. The current year is 2001, and society is in a chaotic mess as the unpredictable threat comes ever closer. Some scientists predict that the wave will pass harmlessly. Others predict that it will tear the Earth apart. Frankly, no clear consensus can be reached simply because over the past four years no theoretical explanation has come forth to explain the completely incomprehensible phenomenon that created the wave.

A subtle and sometimes ignored element of this threat is the precision path of the filament. It's crossed 25 light years with pinpoint precision on Earth. Such an event happening by chance is so unlikely that very serious scientists see this event as nothing less than an intentional attack by an alien force.

The actual effect of the wave is left as an exercise for the reader. Here are some potentials:

1: Global geomagnetic storm of cosmic proportions, paired with heavy charged particle bombardment. Huge lighting discharges all over the surface, and even deeply buried conductors undergo extreme magnetic flux.
2: Cosmic ray torrent, irradiating everything on the surface for over 20 hours.
3: Change in fundamental constants in the presence of the wave, causing all solar system bodies larger than a few hundred km across to break into fragments.
4: Atmospheric ionization strips off 60% of Earth's atmosphere, resulting in chaotic storms before finally stabilizing just below a safe level for long-term survival at sea level.
5: Solar disruption resulting in a more-or-less constant flare state. Less intense than #1, but still more intense than anything previously recorded and continuing for the forseeable future.
6: Magnetic field inversion. As the wave passes, like charges attract and dissimilar charges repel. Scientists would be able to perform interesting experiments in the picoseconds before every atom on the planet disintegrates.
7: Antimatter enrichment. The wave passes with minor magnetic disruption, but leaves a thick sleet of stable antimatter clusters throughout the solar system.
8: The asteroid belt is disrupted, causing a new era of asteroid impacts throughout the solar system.
9: Ozmic enrichment causes space, but not the surface of any planets, to become a very-high-mana environment.
10: Psion resonance causes a chaotic projection wave to take place. Briefly, all humans astrally projected, and then returned to bodies. Some 20% of people returned to their own bodies, another 60% returned to other people's bodies, and the last 20% were apparently carried away with the storm... and there are rumors that some number of bodies were inhabited by alien minds as well...
11: The wake of the storm disrupts all electronic devices, creating a world not unlike Rustic.
12: 99% of the population disappeared in classic banestorm activity.
13: For 8 hours at the peak of the storm, all human beings entered a dream state and collectively experienced life as if they had been transported to an alien homeworld; they were informed that this was the only way to communicate long-distance and given instructions for a device that would, when completed, trigger the Sun to detonate in a way that would safely transport all humans to the aliens' advanced home world.
14: For 298 hours at the peak of the storm, all humans experienced inarticulate howling and screeching that linguists struggled to comprehend. The entirely psionic noise could not be stopped and prevented sleep. Though some people apparently died more-or-less directly from sleep deprivation, far more died from secondary effects, attempts to promote sleep, and attempts to deafen themselves. Those that survived and attempted to record the message found that it was structured like language, but records were almost impossible to make and incredibly inaccurate after the first ten hours.
15: Millions of ablative-coated eggs rain down onto Earth, eventually hatching into apparently non-sapient five-fold symmetric sea creatures that proceeded to consume nearly all multicellular life in the oceans.
16: All artifacts and chemical traces of humanity vanished instantly, leaving nothing but naked humans, some of them suddenly missing important parts. Those in tall buildings plummeted, those in underground facilities were fossilized. Domesticated plants and animals were not destroyed, but often redistributed or otherwise disrupted by the terrain reconfiguration.
18: Intense neutron radiation caused many nuclear warheads to spontaneously detonate, and all reactors and most waste piles to melt down.
19: Nothing happened, despite desperate warnings. The wave simply passed by unnoticed except for a gentle planetwide aurora.
20: A huge number of objects were transformed into reality shards of all of the previous possibilities. Grasping the object transports the holder to the reality in question so long as the item is held. The only warning is that the object has a faint feeling of static repulsion.

[Flyndaran]

That raises the question of whether it counts as a Lucifer if the disaster is predicted but hasn't occurred yet.

[PTTG]

True.

Here's another one.

A Reich parallel where the victorious 1000-year dream falls apart only months after it's founder's death. The chaotic world that remains gradually collapses to warlordism around the globe, even in a Pelley-ized USA. Biological and chemical warfare is used as a matter of course in this 2020-era apocalypse. Somehow, it managed to avoid ever inventing working nuclear bombs, although radioactive substances are sometimes used as area denial tactics.

It's practically cozy compared to some previously-mentioned catastrophes, but it's still a harsh and horrible life for anyone who's not a warlord.