My story is an urban fantasy tale, set on an alternate Earth where every mythical humanoid that humanity ever thought was real, is in fact real, or at least used to be before going extinct. From vampires and werewolves to elves and dwarves to merfolk and minotaurs, they all came into being because at some point in history, a magically-potent human under the right conditions spontaneously mutated into the first of a new magically-active species that then created more of its number by breeding with humans. Due to their immunity to aging and disease, they are collectively referred to as immortals. However, they very much can be killed.

The reason these species haven't overrun and dominated the human race is because, even before they were forced into concealing their existence in the early 1700s, my setting has a phenomenon that keeps their numbers in check: moontime.

Every lunar cycle, at the moment the moon reaches peak fullness, time stops for everything non-magical for a full 24 hours, and the world is flooded with magic. However, for reasons unknown, moontime has an odd effect on areas that have seen excessive immortal foot traffic. If the monthly average immortal population of an area exceeds approximately 1 in 1,000 humans, then for the duration of moontime that area will find itself beset by horrific, otherworldly abominations with an appetite for immortal flesh. Furthermore, the higher this ratio gets, the more intense the onslaught the immortals will have to survive. So immortals eventually figured out that they need to keep near human settlements, blend in, and never let their numbers exceed 1 in 1,000.

But sometimes, surviving moontime isn't just a matter of the immortals keeping their own numbers down. It's about the humans keeping their numbers up.

It occurred to me that many disasters would disproportionately affect humans in their death toll, most notably plagues due to the immortal population's immunity to disease. Any mass die-off of the human population would leave the immortals as an uncomfortably high percentage of the population.

This results in a tragedy known as a "Reaper's Moon". A month where, whether culled by the moontime monsters at the end of it, or slaughtered by their own people to prevent the former, a huge number of immortals die to bring the population back in line with the recently-reduced human population. And they're remembered by those who were there as horrific and harrowing tragedies where friends were forced to turn on each other, or else unite against unspeakable horrors.

I initially imagined that one of the most famous Reaper's Moons would have happened to the Native American immortal population amidst the plagues following the Columbian Exchange, but looking at estimates of the number of deaths and the timescale over which they happened, it was far too slow and steady for any specific month-long period to have a memorably hefty death toll. The moontime deaths of that period would be remembered more collectively, as a long, slow and painful series of smaller cullings that gradually wore them down to almost nothing.

No, if I want there to be a specific, individual Reaper's Moon that the immortals who survived it look back on with horror as the single darkest moment of their centuries-long lives, I need something else entirely.

What historical tragedy, one which would logically disproportionately affect humans as opposed to magical creatures with enhanced strength, speed and endurance, would result in a massive and sudden shift in the area's immortal carrying capacity and result in an infamous and horrific Reaper's Moon?

  • $\begingroup$ This event can happen anytime, anywhere? What's the boundaries, if there are any? $\endgroup$ Apr 27, 2022 at 13:55
  • 1
    $\begingroup$ @Tortliena The only restriction I can think of, aside from the fact that we have to have the historical records to know it happened, is that the mass die-off of the humans in the area had to happen in the time-span between two separate full moons. $\endgroup$ Apr 27, 2022 at 13:57
  • $\begingroup$ The plagues introduced to the Americas that you use as an example didn't wipe out communities within the span of a month. Smallpox did kill hundreds of thousands of people but it did so over the span of years, for instance it took two years for a smallpox outbreak in Plymouth to reach Lake Ontario. $\endgroup$
    – sphennings
    Apr 27, 2022 at 14:02
  • $\begingroup$ @sphennings Yeah, like I said, I later realized they wouldn't work for a Reaper's Moon for that exact reason. $\endgroup$ Apr 27, 2022 at 14:04
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    $\begingroup$ @sphennings That's why they bar it out for their moonster reaping reaper's moon :). Too slow to be a good trigger ^^. $\endgroup$ Apr 27, 2022 at 14:04

7 Answers 7


One of history's great famines

Initially, I was going to suggest something like the Rwandan Genocide, or the Firebombing of Tokyo, but as you note, the immortals can be killed, they just don't die of natural means. Assuming that also covers starvation, the most brutal Reaper's Moons would happen during times of intense famine, when everyone around the immortals would be dying off en masse. Thematically, this is even quite evocative and appropriate:

for the duration of moontime that area will find itself beset by horrific, otherworldly abominations with an appetite for immortal flesh

remembered by those who were there as horrific and harrowing tragedies where friends were forced to turn on each other

These are characteristic traits of the worst famines, where starving families will kill their own children to stave off hunger, and when the people around you become the very monsters you fear.

It will likely be doubly bad because if the starving people around you notice that you look pretty well fed, they might become the very abominations you fear.

Some potential candidates:

Some of the worst Soviet famines:

  • The 1921-1922 Russian Winter famine. 5,000,000 dead in the Volga region over the course of a 6-7 months, or a ~million a month
  • The 1930-1933 Holodmor. 7,000,000 dead over three years. (good choice for historical/cultural cachet)

One of the catastrophic Indian famines during British rule in India
(many of these are poorly documented and might have affected more people)

  • The Great Bengal Famine, 1769-1770, 7-10,000,000 dead
  • The Chalisa Famine, 1783-1784, >10,000,000+ dead, entire areas depopulated
  • Doji bara famine (The Skull Famine), 1791-1792, >10,000,000 dead

One of China's worst famines in the 1900s:

  • The Great Chinese Famine, 1959-1961. Potentially 50,000,000+ dead.
  • The 1907 Famine. Potentially 25,000,000+ dead of starvation and violence
  • $\begingroup$ There's a requirement that all deaths take place in a 1 month window. More precisely between two full moons. $\endgroup$
    – sphennings
    Apr 27, 2022 at 15:47
  • $\begingroup$ @sphennings true, but that is still a lot of deaths over a much smaller timescale. Definitely something to consider. $\endgroup$ Apr 27, 2022 at 15:55
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    $\begingroup$ @sphennings 25,000,000 people dead over a single winter is 5-7 million dead a month. Outside of famines, hard to find something that kills that many people in that short of a timescale. $\endgroup$
    – Daniel B
    Apr 27, 2022 at 16:13
  • $\begingroup$ It looked odd that all 3 Indian famines had the same death toll - the same estimate of 11+ million is coincidentally listed for both Chalisa and Doji bara famines, but the Great Bengal Famine estimates are 2.1-3.8 million. $\endgroup$ Apr 27, 2022 at 16:17
  • $\begingroup$ @KerrAvon2055 "Between seven and 10 million in conventional estimates; revised down to between one and two million in some recent scholarship". I've edited the answer to remove the unlikely-feeling coincidence. $\endgroup$
    – Daniel B
    Apr 27, 2022 at 16:23

I think that It might be a mistake to dismiss the post-Columbian epidemics in the new world as being too slow.

They caused a massive population decline in both continents which lasted for centuries among the native - the Indian populations probably didn't stop declining and start recoving for about 400 years until about 1900. So the overall rate might be too slow for a spectacular event.

But some of the many, many individual epidemics which composed this continent wide centuries long population decline would have been very catastrophic.

The Mississippian culture was a Native American civilization that flourished in what is now the Midwestern, Eastern, and Southeastern United States from approximately 800 CE to 1600 CE, varying regionally. It was known for building large, earthen platform mounds, and often other shaped mounds as well.1 It was composed of a series of urban settlements and satellite villages linked together by loose trading networks.2 The largest city was Cahokia, believed to be a major religious center located in what is present-day southern Illinois.

The Mississippian way of life began to develop in the Mississippi River Valley (for which it is named). Cultures in the tributary Tennessee River Valley may have also begun to develop Mississippian characteristics at this point. Almost all dated Mississippian sites predate 1539–1540 (when Hernando de Soto explored the area),3 with notable exceptions being Natchez communities. These maintained Mississippian cultural practices into the 18th century.4

The Mississippian culture was a Native American civilization that flourished in what is now the Midwestern, Eastern, and Southeastern United States from approximately 800 CE to 1600 CE, varying regionally. It was known for building large, earthen platform mounds, and often other shaped mounds as well.1 It was composed of a series of urban settlements and satellite villages linked together by loose trading networks.2 The largest city was Cahokia, believed to be a major religious center located in what is present-day southern Illinois.

The Mississippian way of life began to develop in the Mississippi River Valley (for which it is named). Cultures in the tributary Tennessee River Valley may have also begun to develop Mississippian characteristics at this point. Almost all dated Mississippian sites predate 1539–1540 (when Hernando de Soto explored the area),3 with notable exceptions being Natchez communities. These maintained Mississippian cultural practices into the 18th century.4


It is believed that pigs from De Soto's expedition introduced diseases which rapidly reduced the populations of Mississippian communities. The largest Mississippian communities would have been large enough for a few immortals to live as 1 in every thousand inhabitants.

The Cahokia Mounds State Historic Site /kəˈhoʊkiə/ (11 MS 2)1 is the site of a pre-Columbian Native American city (which existed c. 1050–1350 CE2) directly across the Mississippi River from modern St. Louis, Missouri. This historic park lies in south-western Illinois between East St. Louis and Collinsville.3 The park covers 2,200 acres (890 ha), or about 3.5 square miles (9 km2), and contains about 80 mounds, but the ancient city was much larger. At its apex around 1100 CE, the city covered about 6 square miles (16 km2) and included about 120 manmade earthen mounds in a wide range of sizes, shapes, and functions.4 At the apex of its population, Cahokia may have briefly exceeded contemporaneous London, which at that time was approximately 14,000–18,000.6

The population of Cahokia declined during the 13th century and it was abandoned by about 1350. So a nuclear family of immortals who found that theywere safe in Cahokia might have had to face death as the Cahokian population declined.


Similarly, early explorers report dense concentrations of villages along long stretches of the Amazon River, while later explorers reported the region was very thinly inhabited.

There were much larger cities in Mesoamerica and the Andes of South America. And Spanish colonists recorded various epidemics among the Indians in those cities in the centuries after the conquests of those regions. A specific epidemic which reduced the population of a large city by 25 percetn or 33 precent or 50 percent would have meant disaster to a group of immortals there who were close to the upper limit before the epidemic.

And of course there have been deadly epidemics in various old world cities too. Constantinople had a population of hundreds of thousands whentheplaguebrokeout in 541-549.

Procopius,[11] in a passage closely modelled on Thucydides, recorded that at its peak the plague was killing 10,000 people in Constantinople daily, but the accuracy of the figure is in question, and the true number will probably never be known. He noted that because there was no room to bury the dead, bodies were left stacked in the open. Funeral rites were often left unattended to, and the entire city smelled like the dead.[12]


If during the plague over a thousand died each day for a year, the population of Constantinople would have declined by 365,000 duirng that year. And if the immortals in Constantinople were almost one in ever thusand, almost 365 immortals would have had to leave the city amoung the crowds of mortals that abandoned it. Each immortal would have had to select a destination town with a population of at least 2,000 to hid in, so that there would be enough people left over after the plague devastated that town, and hope that no other immortalwa s headed for that town.

So you need to look up the most populous cities in history, especially the most populous cities before the rise of modern medicine and sanitation, and then look up the major plagues which reduced their populations at avious eras.

What about the Chicago Fire in 1871? Comparatively few people were killed, but tens of thousands lost their homes and became refugees. And the Peshtigo. Wisconsin fire started on the same day and killed far more people, probably over 2,000, and thousands of others had to flee from the fire.

And what about the times when a large city with a vast population was captured by an enemy.

Vijayanagara was the large capital city of the Vijyananagara Empire in the 16th century.

The city was a powerful urban centre in South India from 14th to 16th century and one of the ten largest cities of the world.

An ongoing war between Muslim Sultanates and the Hindu Vijayanagara Empire led to the Battle of Talikota in 1565 CE, fought about 175 kilometres (109 mi) north. It resulted in the capture and beheading of Vijayanagara leader Aliya Rama Raya, mass confusion within the Vijayanagara forces and a shock defeat.[13][32][30] The Sultanate army then reached Vijayanagara, looted, destroyed and burnt it down to ruins over a period of several months. This is evidenced by the quantities of charcoal, the heat-cracked basements and burnt architectural pieces found by archaeologists in Vijayanagara region. The urban Vijayanagara was abandoned and remained in ruins ever since.[33][16][34] Vijayanagara never recovered from the ruins.[31][35]

The Italian Cesare Federici writing two years after the empire's defeat states that "The Citie of Bezeneger (Vijayanagara) is not altogether destroyed, yet the houses stand still, but emptie, and there is dwelling in them nothing, as is reported, but Tygres and other wild beasts."[36]

Sanjay Subrahmanyam states that Vijayanagara was arguably one of the only three centers during this period with a population of over 100,000 in South India and that from the contemporary accounts and what remains of its expanse, the city proper and the suburbs had a population of 500,000 to 600,000. He notes that Domingo Paes had estimated its size at 100,000 houses.[40]

So in the mere months of the Sack of Vijayanagara its population may have plummetted by over 500,000 as former inhabitants became fleeing refugees, slaves dragged away in chains, or rotting corpses. And if the immortal population had been close to the limit, over 500 immortals might have needed to find new homes in other cities or die.

And of course other cities may had even higher populations that Vijayanagara and been massacred by rutheless enemies.

The Mongols were notorious for massacuring the entire populations of cities they captured. In most cases those cities would have had only tens of thousands of inhabitats, room for only a small population of immortals.But a few had populations of hundreds of thousands. The Mongols also drove the rural population, much larger than that of the cities, to the cities, to labor for the Mongols in the sieges and provide human shields for attacking Mongols. And I expect that the country peopl wold also have been massacred after the Mongol victories.

So there were reportedly hundreds of thousands and even sometimes over a million persons reported killed at some Mongol sieges, though some historians consider those numbers exaggerated.

One very bloody day during the Mongol conquests was March 19, 1279, when the Battle of Yamen completed the Mongol conquest of Song Dynasty China. Thousands of Chinese soldiers were killed, but the majority of deaths were civilians, and at elast 100,000 are believed to have died on that day.

I note that possbily some immortals might find themselves in a large moving group of people, an army on the march, or a migrating horde of barbarians, or a stream of refugees. And if many of the others in the group died or were killed, the immortals might suddenly find that they were way more than 1 in a thousand.

Here is a link to a list of deadliest natural disasters.


So you need to find the bloodiest disasters and their dates. And then you need to find the lunar phases at the dates of those disasters. Since the "Moontime" when the immortals get slaughtered if their population percentage is too high is at the instant of fullest full moon, you need to find really big disasters that happened hours or minutes before the instant of full moon. You don't want a significant percentage to the deaths to happen after the full moon, but you don't want the deaths to be long enough before the full moon for the immortals to start to migrate to other highly populated regions.

And astronomy programs should be able to giv e the dates of the full moons closest to the various terrible natural disasters in history.

For example, here is a link to an online moon phase calculator.


And here are links to lists of bloodiest single days in military history:





The major bombings of cities include:

Dresden 13-15 February 1945, 25,000 dead.

Nagasaki 9 August 1945, 39,000 dead immediately.

Hamburg 24-30 July 1943, 42,600 dead.

Hiroshima 6 August 1945 50,000-60,000 killed immediately.

Tokyo 9-10 March 1945 80,000 to 130,000 killed.

And probably many thousands also fled from each of those cities, dropping their populations even more.


The Battle of Cape Ecnomus in 256 BC might have been the largest naval battle in number of men present. The Romans allegedly lost 10,000 killed and the Carthaginians lost 30,000 to 40,000 killed and captured, making it a very bloody day and similar to the Battle of Salamis.


And psosibly you might want to consider changing the maximum proportion of immortals in a population so the immortals have higher population levels and so suffer more fatalities during moontimes following various disasters.

  • $\begingroup$ You got bit of a repeat with the Mississippians, but all very good points $\endgroup$
    – No Name
    Aug 20, 2022 at 20:05

Don't need to kill many people if there's population migrations

Catastrophes and wars make deaths, obviously. But it also moves people. Looking at the current war in Ukrainia, more than 5 million people were reported as refugees, that's a whopping 10% of Ukrainia's population. But more interestingly, the movements are not made uniformly, they create a depression outwards the conflict zones. Such turbulences are your best bet in creating reaper's moon events. Indeed, while every immortal's life was organized and stable beforehand, they're now forced to move into others territories, and :

  • Nothing guarantees that their destination has enough population margin to prevent a reaper's event.
  • Nothing guarantees that they'll spread out uniformly among possible destinations.

Indeed, since you cannot really order each refugees where they have to go (it's chaos and they're concealed, after all), you'll quickly reach over-saturation in places and under-saturation in others. It's like when you throw a coin 1000 times, it's almost never exactly 500 heads / 500 tail, it'll most likely deviate by 5, 10 or even 30, enough to reach the critical thresholds you want to hit.

Besides, in case of wars where people who can fight are needed, you'll remove the fragile population (elders, kids...) from the frontline, leaving most likely the immortals who will be enrolled and gathered into the same place. In other words, you cull part of the population while keeping the same amount of immortals at the same place, increasing concentration.

An example

We'll look at small or medium cities in the order of 20000 to 100000 people. The critical threshold would be around 20 to 100 immortals, at which point a reaper's moon can occur.

Now let's say you already have 1 immortal for 1200 humans -a nice 20% above your limit-. You'll have between 16 and 83 immortal pop'. As such, you only need 4 more immortals to reach the critical threshold in the smallest town and 17 in the bigger ones. If we add in half of the population as human refugees, you'll need only half more to saturate, that's still a very low number, statistically speaking! We're not counting in the hundreds, but in the tens! It's very liable to local randomness.

This low values can -and will- lead to some towns having a few immortals too much coming in while others cities could accept more, enough to trigger the event in some -if not many- places.


Most big wars and cataclysms which move lots of people around in short amounts of times are good candidates. Ukrainia's invasion, World wars 1 and 2, tsunamis and earthquakes (like the 1923's Kanto's one)... All of these are very likely to create a reaper's event, even if we forget about casualties.

  • $\begingroup$ Speaking of WWII, you'd better bet that these immortals would be targeted by the holocaust, Not directly, of course, but the whole point of the Holocaust was to destroy perceived outsiders - Jews, Roma, homosexuals, the mentally ill - all of which had members that might pass and were scared enough to try, analogous to the immortals. Those concentration camps would round up plenty of immortals, and I wouldn't be surprised if there were several Reaper Moons over the course of the Holocaust. $\endgroup$
    – No Name
    Aug 20, 2022 at 20:01

Black Death

There was a bout of plague in Europe that started around 1346. Over 7 years, it killed between 75 and 200 Million people.

140 Million people over 84 months gives about 1.6 Million people per month. Assuming any kind of seasonality to the plague - eg, flea and rat populations peak in spring/early summer - you could easily see months with significantly higher death tolls.

It is estimated to have killed between 30% and 60% of Europeans, so the immortal population would have to decline by the same amount over the same 7 years.


1918 was a bad year.

It's the last year of WWI and the "Hundred Days Offensive" - which only covers the Western Front from August until the armistice - saw 2 Million casualties. (mostly wounded, but we're still looking at hundreds of thousands of dead.)

Two major epidemics break out: the Spanish Flu kills 17M-50M over 2.5 years, and the Russian Typhus epidemic kills 2.5M over 4 years.

Russia and Finland are both having civil wars in 1918, and Russia is also fighting in Ukraine.

For your immortals it might be worse: mass mobilizations are required to support all this fighting, so even the US (which saw comparatively light casualties) might see major Reapings because so many people have left home to support the war.


Slaughter at Jericho

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. . . or any historical battle where one army slaughters the entire population of a town. Ideally one more recent than Jericho.

In the Biblical battle of Jericho, the Israelites march around the walls seven times, carrying the Ark of the Covenant and blowing their trumpets. On the seventh day they shout loud enough to make the walls fall down. They then go inside the city and put every man, woman, and child to the sword. Or so they thought.

In fact there were many immortals living in the city who escaped the sword. You see Immortals always know a way out. They always have an escape route and somewhere to hide, in case the locals decide they don't like immortals any more and form a lynch mob.

The Immortals' secret tunnels and hiding places kept them safe from the murderous Israelites. In this regard the Biblical account of the Siege of Jericho, in particular Joshua 6:21, is inaccurate.

Once the Israelites slaughtered all the mortals and grabbed everything they could carry, they abandoned the corpse of Jericho. This left a city full of immortals ripe for the next Reaper Moon.

Homework: How do the Israelites react to the sudden horde of nightmarish monsters in the city? Are they sent by God to enact his wrath on the unbelievers. Or are they sent from the unbelievers' heathen gods? Remember the Israelites did believe in other gods such as Moloch. They just thought their own god was more powerful and righteous.

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    $\begingroup$ The population of Jericho at that time is estimated at 2,000 (so, 2 immortals). Ancient cities just couldn't get all that big. $\endgroup$ Apr 28, 2022 at 0:39
  • $\begingroup$ @OwenReynolds Hmm. . . you're right. I don't know why I got it in my head we needed everyone but the immortals to go away. We only need to get rid of enough to turn 1 in 1005 immortals to 1 in 999. $\endgroup$
    – Daron
    Apr 28, 2022 at 13:32

It is not so easy to keep count of the mortal/immortal ratio

Similar to Tortliena answer, immortals need to keep a 1/1000 ratio on the monthly average immortal population, and I think the same can be said about the mortals

This means that in case of immortals who just stay a pair of day in a place - unbeknownst to the local immortals - they could tip the monthly balance of the area. Even because I expect the immortal/mortal ratio to be always quite near the threshold (it is easier to "cumulate" immortals rather than immortals because of the longer life span).

So, some possibilities that could trigger a reaper moon even in absence of a big event (in areas where the ratio is dangerously near the culling threshold and the immortals aren't able to calculate exactly the average ratio)

  • A big people shift (pilgrimages, concerts...) of some days during the month tilts the ratio and the local immortals weren't able to realize it before it is too late
  • Some travelling immortals stop in a city for 2-3 days without alerting the local immortals (they don't need to be in group, maybe they've been to the city independently)
  • A city loses enough mortal population in the summer (because of vacations) to go below the safe ratio

People could argue that immortals are quite naive in remaining in a city/place in these ambiguous situations, but it should be remembered that even moving away would require the knowledge of a safer place, so in some cases they would have no choice but to stay and hope for the better

Note that this could also be weaponized by some immortals, so I think that they would try to enforce some rules against free movement to avoid this.

  • $\begingroup$ I guess Woodstock makes the list then, at least for places that aren't Woodstock $\endgroup$
    – No Name
    Aug 20, 2022 at 20:11

The aftermath of A War

There were multiple population shifts after the first and second world wars. Maybe instead of doing one massive event there were multiple "Reaper's Moons" across different countries and wiped out a lot of the immortal communities in those areas.

You could do it that some of them went of to fight or hid at home, depending on which one it could be that they head to spread out due to most men going of to fight. Or it could be that they were still mending from the war and didn't realise that they lost so many people in their local communities. This way they're hit twice within a short span of time and would cause them to be horrified of the time as well as being cautious of future wars.


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