Many fantasy books and campaign settings seem to include some form of huge world changing event such as a huge Meteorite strike. One of the classic examples from the game world would be from the Dragonlance Setting where the City of Istar was struck by a "Fiery Mountain" when the Kingpriest of Istar offended the gods by demanding he be given godlike powers to eliminate evil from the world. Many campaign settings (my own included) have featured some form of cataclysmic event in the past that had shaped a world where ancient temples and cities lay uninhabited and rich for the picking.
Putting such an event firmly in the past can help shape the world in which the PC's adventure, but why not make the Cataclysm an adventure in itself? Of course the effect on impact (and the aftermath) will be determined by many different factors such as the objects size, composition speed, angle of entry, whether it hits land or sea and the composition of the underlying rocks.
The first thing the GM needs to do is determine the size of the impactor as this will effect the scale of the disaster and shape the after-effects. A solid metal Meteorite for example would likely remain largely intact until impact but would expend all its energy excavating the target point. If this were over the sea huge Tsunamis would be formed and if over land continent blotting dust cloud's created.
A comet may leave less of an impact crater but would expend more energy in the Atmosphere creating huge shock-waves and atmospheric disturbance. Recent scientific studies have shown that much of earth's water and organic molecules may have been deposited here by comet impacts early in the life of the planet.
Asteroid's used to be thought of a huge chunks of rock, and some may be just that, remnants of ancient planetary collisions. However many asteroids are probably loose collections of boulders and dirt held together by their own weak gravity. This does not mean that they would not pose a serious threat of one were to strike the Earth's atmosphere. An Asteroid would likely break up as it approached our planet and be torn apart by the tidal effect of our own gravity. But this could mean that instead of a single cataclysmic impact the planet would be bombarded by smaller, but no less deadly, impacts over several hours or days. In July 1994 scientists observed the comet Shoemaker-Levy 9 break up in just this way before slamming into Jupiter in over 20 separate impacts.
Even if the incoming object was not large this does not mean it could not be deadly. A small asteroid (30-200 feet across) for instance would slow down rapidly upon hitting our atmosphere. The compressed air in front of the incoming object would be heated to incredible temperatures and the pressure exerted on the meteor would heat it up. This pressure can build until it breaks the meteor into smaller parts which would themselves heat and break... in effect the whole rock would explode. Just such an event occurred in 1908 over Tunguska in Siberia. The rock that entered the atmosphere was about 150 feet across and when it exploded it dumped energy equivalent of 20 million tons of TNT (100 Hiroshima's) into the atmosphere. The resulting airburst flattened and scorched hundreds of square miles of woodland. Had this occurred over a major city the loss of life would have been unimaginable.
Physical effects of an Impact
A 1km stony asteroid striking the atmosphere would explode with the equivalent of 100,000 megatons of TNT. This would create a crater 24km wide and would burn all combustible material within a 150km radius almost instantly. A few seconds later the shock wave would create 500km/h winds to a radius of 130km destroying buildings and ripping up trees. Trees would still be flattened and many building structurally damaged up to a radius of 300km. Anything left standing would likely be shaken to the ground by an earthquake that could measure up to 8 on the Richter scale. Regional climactic changes brought on by dust and soot would cause a localised 'nuclear winter' and increase acid rain, resulting in crop failures for several seasons on a continent wide scale.
A larger impactor, such as a 5km wide asteroid would create global problems. The initial impact would explode with the equivalent of 10 Million megatons of TNT leaving a crater 10km wide. Fires would be started up to 600km from the impact and buildings damaged by the blast up to 1100km away. Earthquakes would be felt across the whole world and the effect of dust and ash would plunge the world into darkness for months. Most plants would die and it would take decades for significant recovery resulting in major global extinctions and massive population decline for all species. Ironically after the global nuclear winter there might follow a period of rapid global warming as billions of tons of vaporised CO2 send the global climate into hyper drive. (Source: Earth Impact Effects Program)
Effects on a Fantasy Setting
Modern society may be better able to survive a huge impact although the long term effects might have drastic consequences on the economy and ecology of a technologically developed civilisation. A pre industrial society however would likely suffer extreme upheaval and social breakdown. On a medieval society - with little or no social support systems in place - the consequences of a large impact would be truly cataclysmic.
Given the poor construction of most buildings during this time period it is likely that many would collapse either as a direct result of the blast or in the subsequent earthquakes. Some better constructed wooden buildings may survive if they are outside the thermal radiation zone. However with pretty much everything else around them in ruins the chances of city wide fires would be very likely. A large enough impact (such as the 5km asteroid described above) could wreak havoc across the whole globe. Outside the cities life would not be much better as crops failed, livestock starved and everything succumbed to thick choking dust.
Under such circumstances any semblance of government would quickly become meaningless and civilisation would become brutal and primitive. The existing rules of society would quickly break down and for a time at least anarchy would reign. Slowly as the survivors gathered together 'natural law' - simple, brutal and harsh - would develop and become the standard.
However fantasy setting has one thing that the real world does not - Magic. It's possible that Mage's and clerics would quickly rise to high status in ravaged society as their skills might mean the difference between life and death for a small tribe. The other alternative - in a low magic setting where Mage's were few to start with - is that they become highly sought after commodities, bought and sold by local warlords. Clerics in particular might be highly prized. Imagine what a cache of antibiotics and other drugs would mean to a small group post apocalyptic survivors from the modern world. It would give them an edge, a chance for survival, that their ancient ancestors lacked. Although both groups might be on parity in terms of resources the residual technology (in this case medicine) would give modern survivors a much greater lifespan and a chance to rebuild. Such would also be the case to survivors of a fantasy society possessing a cleric/healer or a Mage.
One possible side effect of a cataclysm like this would be the rise of religions and cults not previously tolerated. In my campaign world the cult of the Manifester's grew out of a cataclysm brought on by magic. They eschewed all magic and were responsible for many public acts of violence against Mage's. Another alternative would be cults that blamed particular groups of society for the disaster. Logic does not have to be part of the reasoning of these cults, all that is required is that the target of blame is different in some way. All kinds of weird and wonderful cults might spring up, and of course in a world where evil scheming devils are a fact of life, there are a lot of powerful beings who would not pass up the opportunity to become the focus of these bizarre cults.
Whether you place your PC's in the midst of an unfolding cataclysm or use it as the backdrop to the culture they grew up in the possibilities are literally endless.