Thanks for all the great responses so far, guys. I’ve expanded the list based on ideas from Sleeper (rpg.net), mtferris (rpg.net), mindstalk (rpg.net), Redbeard67 (rpg.net), Paraxis (ENworld), sckeener (ENworld), PeterWeller (ENworld), Castlin (ENworld), mrtomsmith (ENworld), smorgasboard of pie (WOTC boards) and LordZordran (WOTC boards). Apologies to anyone whose name I’ve forgotten or misspelt.
9. BLACK MAGIC. One of the jobs of the medieval inquisition was to find and route out witches and sorcerers. Black magic was seen as a terrible corrupting influence, because it was useful (helping with childbirth or to curse a rival), but inevitably brought a community closer to evil. In the fantasy medieval world, of course, black magic is very real, and if it isn’t stopped it can easily turn a point of light into a flame of darkness.
10. INFIDELS AND HERETICS. The real middle ages give us a sad picture of religious war and oppression, but in the fantasy medieval world heretics and infidels may genuinely worship dark corrupting gods. Even worse, they may convert members of the communities in the points of light, bringing them over to the worship of the dark gods so that they eventually become part of the darkness themselves.
11. BAD BLOOD. The real middle ages were a story of constant warfare. Christian against Christian, Muslim against Muslim, neighbour against neighbour. The fantasy medieval world is no different. Envy of natural resources, ancient claims on a neighbour’s throne, trade disagreements, territorial disputes and the like can all lead to one point of light undermining or even going to war with another. Of course, in the fantasy medieval world, devils will be hard at work helping this process along.
12. STRANGERS. Strangers were unusual in the real middle ages (since travellers were few) and were greatly feared. Lords passed laws against peasants harbouring strangers. In the fantasy medieval world, strangers are even more dangerous. Is that group of merchants bringing vital supplies trustworthy? Should we let them through the gates? Or are they a pack of werewolves, just waiting for night to fall so that they can shapeshift and get at the townspeople’s throats?
13. FAMINE. Food storage systems were not very advanced in the middle ages, and what surplus was produced was often sold off for cash by the lord. If there was a bad crop, people would go hungry. Two or more famine years in a row would spell complete disaster. Unfortunately, in the fantasy medieval world, there may be dark sorcerers, malicious fey or other powers actively working to bring on famine. And the terrible danger of travel means that bringing in food from outside won’t be easy.
14. PESTILENCE. Infectious diseases were horrendous killers in the middle ages. The Black Death killed between 1/3 and 2/3 of the population of the areas it passed through, and killed somewhere between ¼ and ½ of the entire European population. And in the fantasy medieval world, things are even worse. Plagues may be magical or supernatural in nature, and may even be artificially created through black magic or the worship of dark gods.
15. DOGS EATING DOGS. Of course, the various elements of the darkness will prey on each other just as much as they prey on civilisation. Wyverns will eat raiders, raiders will raid bandit camps and so on. Most of the time, this helps the points of light. However, points of light can sometimes find themselves dragged into conflicts between rival elements of the darkness, just as medieval Poland found itself drawn into the conflict between the savage Teutonic knights and their enemies, the Pagan Lithuanians. In addition, if one element of the darkness defeats enough of its rivals, a much larger and more sinister power may develop. Ghengis Khan started out as just another leader of just another tribe of steppe raiders. Similarly, as strong Viking rulers emerged, the Viking raiding parties of the late 8th century were replaced by the Viking invasion armies of the late 9th.
16. THE ‘LESSER’ EVIL. Sometimes points of light may have to accept the protection of a less than savoury overlord, because he’s the only one that can protect them. And what if there is bad blood between two points of light, one much more powerful than the other? Should the lesser point of light accept the overlordship of a dark power to protect itself?
17. TEMPTATION AND SIN. Let’s face it: humans don’t need an external power to generate darkness. Corruption was rife in the middle ages, from the harbour master to the papacy. Greed, ambition and prejudice can affect otherwise good folk. Add lust and arrogance to the mix, and the communities of the points of light may be perfectly capable of turning the light to darkness all by themselves. And of course, the devils will be there to help them…
18. BEWARE THE IDES OF MARCH. Astrology. The conjunction of planets. Eclipses. Comets. Certain points on the calendar and certain omens herald dangerous times when the shadowfell, the fey world or even worse realms intersect with the mortal world. Sacrifices and prayer will be necessary at these times, lest creatures of the otherworld come for the mortals of this one.
19. THE UNKNOWN. In a world where travel is so dangerous, information is scarce. Is the goblin village you’re sending merchant caravans to still there? Or has it been over-run savage human raiders who will be all too happy to receive your goods? There’s an old ruin 20 miles from the village. Is there something in there that could help protect the merchants on their way? Or will those who disturb the place unleash an army of undead upon the point of light?
20. PLACES OF EVIL AND MAGIC. Long-forgotten wars between sorcerers, gods and other beings of power have left some areas brimming with magical energy that can cause all manner of undesirable effects. There are also ancient places, such as monuments to forgotten gods of evil, that may look harmless to the casual traveller, but that actually harbour great evil. These places can infect travellers who come near, causing them to take the evil back to their points of light, contaminating them.