Goblins are really rare. Solitary. You don’t often see one, and almost never see more than one together. And they’re smaller than you’d think, little more than half-size to a halfling. And emerald green. Yeah, they hide a lot.
But sometimes not, none of that.
Sometimes, when happenstance or outside design brings goblins together in close proximity, something truly remarkable happens. If two goblins are in skin-to-skin contact for more than a day—huddling in a burrow from a predator, perhaps, or trapped—they begin to change. They become gregarious goblins. Doesn’t that sound nice?
Something about that skin-to-skin contact causes a release of something in the goblins. They grow bigger. They change color, from that otherworldly emerald to a contrasting black and savannah tan. Their body plan shifts, becoming longer of limb, lean, whipcorded like greyhounds. And they gather. Something about a gregarious goblin attracts other goblins, solitary and gregarious alike. They hunch together, and the solitaries turn gregarious. The more of them there are, the more powerful and wide-ranging the attraction. This works exponentially, as more gather, more turn, attracting more to gather.
Now you have a plague of goblins. And, hungry, they’re on the move.
A plague of goblins—that’s what it’s called, and what it is—is insensate, insatiate, and inexhaustible. It will carpet the earth and befoul watersheds. It will eat everything more than halfway edible in its path, and it moves at a rolling sprint. As the front of the plague stops to eat clean its environs, the back has already exhausted its own and sprints forward to begin anew. In such way the plague continually rolls forward, denuding everything in its path.
It is just as well that goblins are, mercifully, solitary creatures. A plague may come once every other generation, and none but the elves below the sky and the dwarves below the hills have more than one in living memory.