Idea: Dimension-Traveling Fairy Warlords

The Fair Folk are a conquering people and have been for the many trillions of years that they have existed. They roam from world to world, sometimes exploiting the holes between universes (or even burrowing new ones). As factional conflicts break out the losing side departs for new grounds, and eventually even the winners leave for newer shores, abandoning a wasted world. They are in their third generation as they count it, each generation coming into its time only when all the members of the previous generation have died (which happens only through violence or, very rarely, illness). Whether they developed naturally or were created by others they have not revealed, and their civilization began in a tribal hunter-gatherer state that has changed little, adapting only as necessary to the demands of an empire that stretches across the world.

The Fair Folk appear to be attractive, androgynous members of whatever species the observer belongs to, looking slightly closer to whichever gender the observer feels most comfortable with. Each one looks the same as the next one but as the observer spends more time with particular Fair Folk they begin to acquire, to the observer, slight details that set them apart from the others.

They are able to interbreed with anything living, even plants, and these hybrid children are monstrously hideous. Not because they are more inhuman but because they are less so— their parents are so far beyond the bounds of what our minds can accept that we replace the sight with something else, but the hybrids are close enough that we can see them as they are. Hybrids, unlike their alien parents, do succumb to age, but even so they often live for hundreds of thousands of years.

Because the Fair Folk can only reproduce among themselves every few centuries (they can be considered hermaphrodites but only in our understanding, and really included many other sexes that only manifest in species of other universes, and eight Fair Folk are necessary for reproduction) but can reproduce with others every decade or so, they are quickly outnumbered by their hybrid spawn, which by the time that the world begins to get used up have greatly replaced the previous ecosystem (the Fair Folk aren't exactly picky).

• What if a world already controlled by one group of the Fair Folk is invaded by another faction that is leaving either a lost war or a territory with depleted resources?
• What would a single member of the Fair Folk do if it found itself separated from its peers on another world? Slow takeover? Seclusion? Try to summon its kin?