There is a dragon who zealously guards a large trove of treasure, despite never being able to use it in any practical way.
Usual human thinking would say the dragon is the symbol of greed for that very reason. However, greed is internal, not external, and thus a true "monster of greed" isn't a stingy bastard, but something that specializes in killing stingy bastards, basically a lethal light trap for humans. The dragon isn't keeping the goods for himself, he's shielding the world (and -2 IQ adventurers) from them.
These items are so dangerous (and the dragon so, er, benevolent) that the dragon will go on a rampage, raiding villages left and right and putting himself in exponentially increasing danger, to recover even a single stolen goblet.
Each "cursed" object is actually a sort of "beehive" for a swarm of microscopic (100 micrometers long) machines (think "utility fog"). These machines are what actually causes "the curse", by killing people. (For this question, don't worry about the dragon.)
Given a standard (high) medieval Europe as the setting, but with "demonic" (read: technological) force at play, how can these machines achieve their goal of killing as many people as possible?
- The swarm has to operate under the radar; the more subtly it can carry out its objectives, the better. Ideally, people just mysteriously drop dead after being around the "cursed" objects. Humans figuring out how the "curse" works is bad.
- The swarm needs to keep its numbers manageable. Too many and they either won't be able to hide, or won't be able to power themselves.
- The swarm must feed off of ambient energy without storing it in any significant quantity.
- The swarm needs to be able to carry out its function continuously. Basically, whatever it does, it has to leave the swarm in a condition to keep doing it.
- Units of the swarm can't leave the 20-meter vicinity of the item.