Fairies

This is late, but honestly that should be pretty normal at this point.
This, by the way, is the second time I've had to use an idea I had before this blog started.
This one comes from way, way back - I'm pretty sure I'd already written a story about these fairies when I was fourteen.

 They were called ‘fairies’. They lived in hives, and each of their hives was, broadly speaking, a hive mind. Or rather, several hive-minds… there is no entirely appropriate human word for it. Each hive has hundreds of bodies, and each one has hundreds of minds. It is simply that the minds are not connected to the bodies in any way - a single fairy, escaping from a destroyed hive, is enough to carry all the minds that live there away, and to seek refuge in  another hive.
As a fairy hive ages, the minds in it fracture and split, until eventually when the hive has enough bodies, it will split in two, with around half of the minds splitting off, taking about half of the bodies, and setting up a new hive somewhere else.
Fairy minds, not being tied to mortal bodies, can live theoretically forever. Which means that, in spite of limited intelligence and attention span, the eldest fairy minds are more knowledgeable than any other race.
Which often makes people feel rather guilty about killing them.
You see, the fairy minds never die, which means that unless an entire hive is wiped out, the number of fairy minds will only grow over time. And they become what can probably best be described as ‘acutely claustrophobic’ if there are too few bodies per mind.
And so, the fairy hives are constantly expanding across the surface of the world like a cancer - and the term is not random. Despite their small size, the sheer number of resources required by fairy populations, if they are left unchecked, is simply unsustainable, and they inevitably displace other creatures living nearby.
And so, despite the knowledge they carry, the other civilized races are often forced to cull any fairy hives they come across.
Which has, quite naturally, led to incredible hostility from the fairy towards the other civilized races. To them, the murder of the last member of a hive is unforgivable, meaning as it does that one is willingly snuffing out hundreds or thousands of intelligences.
The kingdoms, therefore, which border lands held by fairy tend to be closely allied - working together to hold back the fairy hordes, and to defend themselves from external threats. And there are external threats. Those further from that border tend to be quick to condemn the practice of wiping out so many lives, especially when they are housed in a fleeing body. And whilst the majority are sensible enough to realise that they would prefer not to have to deal with the problem themselves, it is not uncommon  for someone to use this ethical lapse as cassus belli, and to invade. The alliances between border nations helps to discourage this.
For their part, the fairy rarely unite - indeed, they often fight amongst themselves,  considering such clashes far less serious things than would any other race. But fairy can breed unimaginably quickly, and a single swarm can attack with what seem to be millions of bodies - all directed by a single mind, and without the least fear of death. Such great swarms are not common - but nor are they particularly rare, and it is often a struggle for the border nations to drive them back. Indeed, fairy lands have expanded considerably over the last century or so, despite their disadvantage in military technology.
As a result of the fairies, a significant part of the structure of society depends upon how close one lives to the border. A human, an elf, and a dwarf will often have far more in common than any of the three would have with a member of their own species living a dozen miles closer to the border.
Those societies far from the border tend to be fairly typical examples of the societies of their respective races, and the rare fairy who wants to travel in the lands of other races is generally allowed to, so long as they do not try to settle there. It is said that hundreds of years ago, before fairies were considered so much of a threat, they were one of the most integrated races - hives often sent a small number of their bodies to explore the world, which allowed the entire hive to see and experience new things.
The border kingdoms, meanwhile, are very different. They are heavily fortified places, with a patchwork of architecture - each race working together on those things at which they are best. The citizens, too, tend to have weapons close to hand - especially nearer the border - and most kingdoms require citizens to train with those weapons daily.
Although they are constantly wary of the fairies, not only are relations between the kingdoms far closer than they are further from the border, so to are relations between the races - whilst a human elsewhere in the world might be very suspicious of an elf and of elven customs, one living in a border kingdom  would be likely to treat them with just as much respect as another human would.