An alien horde whose only purpose in life is to consume organic matter and turn it into more hordlings. It will seek to do this as fast as possibile and by whatever means necessary. There is no negotiation with it just as there is no negotiation between you and whatever you eat for lunch1. There is only the absolute need to kill or be killed.
In some settings this may be a weapon of mass destruction - either deployed by a current involved species, the creation of precursors or possibly deployed by a contemporary species that didn't survive their deployment.
Game and Story Use
- "The weapon that ate its creators" can be useful campaign background - as can "the weapon that came too late" - either way the locusts are either rampaging about or, possibly, expended and dormant waiting for some idiot to disturb them.
- Suspiciously homogeneous biology in a sci-fi setting might result from someone sterilising and re-terraforming in the wake of a locust swarm.
- A zombie legion might have a similar role in fantasy - albeit only interested in wiping out local sapient life as opposed to all life.