The Hunt

Once a year, not on a fixed date, not predictable, but roughly once a year, The Swarm comes to the surface during the night to find and hunt prey, which they will either kill right there or abduct to feed to the rest of the hive. This event is known as "The Hunt". After The Hunt they disappear back into the caves along with any corpse they might have caused or even any Swarm member that might have died. Quite simple defenses are enough to keep them out and be safe, but that is the reason everyone has to stay in fortified cities or have a town wall around smaller settlements.