Again and again, we see advice on making our game worlds come alive by focusing on interesting and believable NPCs. The players connect with the world through their interactions, connections, affections, rivalries, and enmities. Today I'm suggesting that another thing that makes the world come alive is when those NPCs drop dead.
If the characters have a home base, but spend a long period of time away from it sailing to dinosaur infested islands and such, the time spent can be emphasized by having NPCs grow older, children get married, babies grow into children and young adults, and especially passing away. Does the fact that Sister Alice could have used her magic to save little Temma when she got the grippe last winter affect future adventure plans? How about that goblin raid in the fall that killed Bobert and Cally...Mighty Kemro's presence would have helped a lot.
Even if the players aren't too far away, a sudden death can change the world in drastic ways. When Good King Ranald passes suddenly and his throne goes to the ne'er-do-well Prince Hingle, it's going to affect the day-to-day business in the kingdom, especially those PCs involved in politics in some fashion. Even losing their regular quest-giver should affect the characters, at least somewhat.
And while a DM might use a death to advance some pre-scripted story, randomly generated deaths can also give ideas for how the world develops, and like other random story elements can provide interesting twists to the one guy who knows nearly everything happening in the world already.
So, how to go about doing this in the game? Since most fantasy games are set in a pre-industrial world, it makes sense to look at pre-industrial real-world demographics. Of course, a fantasy setting can have magic that improves life expectancies in myriad ways, but while magic often affects important characters, it doesn't seem to affect the world very much as a whole in most cases. And even in a fantasy setting, a fast-moving plague can spread faster than clerics of high enough level can cure it.
Here's a life table, breaking the population of Imperial Rome down into age groups and showing the annual chance of death for an individual in that group. The percentage of the population at each age is also pretty useful for world-building purposes.
Percent of Population
in Age Group
of Being Dead
by the End
of the Year
This should be an annual "world upkeep" task, done at about the same time and in the same way as major events (Rules Compendium or AD&D Oriental Adventures, for example) are done. Check each major NPC to see if they die in the upcoming game year, and then use a d12 and d30 (or other suitable random method, depending on the calendar) to pick the month and day that the death occurs.
Also keep in mind the many ways people died back in the day. There was a lot of disease and warfare, but there were also a lot of accidents. People got kicked by horses and mules, without strict building codes buildings would collapse, fire-fighting infrastructure was generally inadequate compared to the risk of wooden buildings and thatched roofs going up in flames, and even small wounds could get infected and threaten one's life.
And if an NPC's number comes up on the dice, but a PC (or even another NPC) with sufficient magic is able to come to their aid, maybe they don't HAVE to die just yet, and resurrection is always a possibility anyway. But nobody lives forever, even in a fantasy world.