I posted about this a year or more ago. One thing you don’t see in campaigns is migrations. Actually that’s not entirely true. The forgotten realms had the dragons move. And there were occasional orc hordes from the north.
But think about a large swarm of man sized spiders that breed incredibly fast. They surge and flow back-and-forth across large stretches of forest or plains. as they hunt out an area they move. Like large groups of army ants.

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