Imagine a blight that killed off the grasses and grains. This is not an unheard off event. Mutant spores, molds, and diseases have killed off a wide variety, even who classes, of plants in the real world. Something that kills off grasses, sounds like science fiction (and the premise has been used in many “end of the world” movies and stories), but it is very applicable to a fantasy world… especially because they are mostly agrarian.
First, the human population will go away. Most human peoples have a society based upon grain. Humans need grain for food, for beer, and for feed. So the lowest level of the human society will soon disappear due to starvation. That will have echos up the social change, as there will be no grain food for others, and very few left to grow other things.
Secondly, without grasses and grains, many herbavores will be starving as well. Most forest and plains herbavores will be without much in the way of food. After a big boom to the carnivore population, (which will then even out), the entire food web will shake and begin to collapse. New nitches will be carved out by animals eventually.
Thus the hunting people (including the Elves) will find themselves without game. Gathering people will have issues as well.
Population near the sea or waterways might do better, as they can have protein and food not based upon grain. Inland societies might trade for sea based food, but there are limits to how far the food can go.
On Arth, this is a non issue. The Biomic nature of the world prevents this from being a world wide catastrophy. There are no microbes on the planet. While there are molds and spores, molds and spores can only exist in their native biomes (normally). Mass die off of plants… and the related species… would be corrected by a biome pylon no matter how many keys had been removed or added. If it slowly became the way of a biome, the disaster would not spread to any other biome.
On Kerren, grasses are not very developed yet. Herbavores munch upon bushy, fern like things. Grassy plains have really yet to develop on Kerren. The Humans who have been trying to develop native grains would be quite put out.
Additional Ideas (3)
The destruction of grasses would likely have little to no effect on sea-life, so fish and other seafood would become a main dietary staple. Any permenant settlements would need to be near a river or other decent sized fresh-water source, such as the delta of a river. They would need the fresh water supply, but the fish population of a lake or other non-oceanic geography would be decimated by the sudden increase in fishing. This also calls into question the problem of diseases caused by an improperly balanced diet. If plant life is killed off, diseases such as scurvy would run rampant though populations.
Unless the grasses grew back relatively quickly, fish would be the main food source, which would greatly limit the population size. Too many people means too many fish being eaten and they'd rapidly be right back in the same boat they started out in with no food. This brings in the issues of population control, which, in a medieval society would be limited basically to abstinance, abortion or killing of the child immediately after birth.
Since most real-world medieval societies abhorred the concept of abortion, as many still do today, it brings in massive moral implications. Some religions believed that the soul entered the body immediately after being conceived, and in a medieval society, damage to a fetus was tantamount to murder. Damage to a fetus was not even allowed in some societies as an effort to save the mother.
The implications of a mass death of food supply such as grains and grasses stretch far beyond anything. The ramifications would be immense and would be a world altering, nay, a world shattering event.
Thus, with Famine--Wild Wastes--The Dark Horse, there was a reduction in the global food supply caused by a change in the weather. It is believed that warring elementalists of great power may have disrupted the natural order. Excessive rains in northwestern Formour & the western half of the Heldannic Confederation caused localised flooding, washing away of topsoil, and a corruption of the water supply. These areas were sparsely inhabited, but the effects of the climate change were far more widespread than were those changes themselves.
In addition to crops being destroyed by floods, many fields were damaged by soil erosion, limiting or even preventing replacement crops being grown after the floodwaters subsided. The groundwater was affected by bacterial infection, which spread rapidly as the diseased population (and animals both domestic and wild) had no option but to evacuate into the waters. The floods also greatly hindered movement, both away from the aflicted area and bringing in emergency supplies. With the area being on the outskirts of civilization, local resources taxed beyond their limits, and the national government not being able to effectively send relief, many turned to banditry when their farms were lost... either to the floods or to other bandits. Of course, not a few of these bandits were not local, but rather opportunists who came there to make a bad situation worse. Further complications arose when the excessive rainfall of one season meant a drought followed, as the normal patterns were disrupted.
With the wild animals being driven out of their homes, and the sudden reduction in food, the animals fled the area. This not only made it more difficult for the surviving sentients, but caused wolves, Hobgoblins, and Changing Folk (Lycanthropes) to come down from the nearby Farreaches in search of prey. This even further caused animals to flee and sentients to die.
The population fled in droves: homes were flooded, food gone, bandits & predators increasing, and the growing sickness, brought the infant mortality rate close to 100%. This sudden influx of people into new areas strained the resources there, which had a shuffling effect of migrating & starving people, and further escalated the problem. As the Kingdom of Formour is very much an agricultural nation, it relies upon a dense network of small villages, roadways, and farms, for its very life. Formour exports grain, grain products, and other foods to most of the rest of Midian. This drove food prices up worldwide--as much as doubled in some areas--and similarly caused inflation in businesses peripherally related to food crops. That is, horses & cattle became more expensive outside of Formour (where prices dropped as they were slaughtered en masse to prevent the poor animals from starvation). Caravans became more expensive as both grains & horses increased in cost, and all major military expeditions worldwide temporarily stopped due to the expense.
The plains & forrests of Formour were the worst for flooding and the loss of grains & other food, but travel in or out was slightly easier than in their northern neighbours in the Heldannic Confederation. There the rougher terrain forced the water to runoff into streams becoming raging torrents, blocking all movement of Human, Dwarf, Troll, or animal. This water had to go somewhere... so it added to the flooding problems further south. The relative isolation and difficulty of travel caused the death rate amongst the Heldanns to be even greater than the Formourians. Only a few communities in the western half of the Heldannic Confederation survived.
To make the inhabitants of Midian suffer even more, I had another far reaching problem for them to face. While everyone was dying in northwestern Norditerre (the northern continent & home of Formour & the Confederation), a large volcano erupted in the Greatsea. This blanketed the upper atmosphere, and caused an unseasonable drop in temperatures worldwide. Couple this with the altered weather patterns (and the precipitating effects of high-atmoshperic ash), and deep snow fell over much of the globe. Some areas do not see snow at all save for a few times a century at best. These were the hardest hit, as they were the least equipped. This only further escalated the problems with a lack of food, an increase in cost, and mass death. (Note: this is player-info, none of the inhabitants of Midian know that the volcano caused the unseasonable and thick snowfall.)
This set the stage for the rest of the Books of the Apocalypse series. Mass graves & entire towns were animated into zombies. Bandits & now-homeless warrior/farmers were banded together into mercenary companies, and these were sent on campaigns resulting in more slaughter, and more corpses. Eventually, many of these were incorporated into a massive zombie army, which ruined cities on two continents. The zombies, increase in necromantic activity (and the interest therein assisting a school for necropolitans & mages), and the increase in numbers & influence of the Shadow Knights, set the stage for Death: The Pale Horse.
The climate changes in Osterre caused the Orckish tribes to be banded together under a single despot, whose pirating efforts further hampered travel. The wandering mercenary & national armies that moved into Osterre to stop him, the military efforts to regain the necropolises from the Undead, and new knightly organisations set the stage for the upcomming War--The Red Horse book.
The new civil changes, including the advent of two new nations, openning of the eastern continent (Osterre) to trade, increase in cetain magics, and sweeping economic changes, will set the stage for the final Book of the Apocalypse, Conquest--The White Horse.