"Let him not breed in great numbers, for he will make a desert of his home and yours."- Planet of the Apes (1968)
Goblin plagues are suffered in the less settled areas of the world, particularly in the lands of Arn, though they have been known in the wilder parts of the Eridan continent, as well. They are more common in places which lie near ancient ruins. In such an infestation, tens, perhaps hundreds, of goblins swarm forth from underground dens or nests. They overrun manor, hamlet, and village, and have even been known to assail the gates of small cities.
No one knows what spawns goblins, but it is certain they don't reproduce in the manner of most humanoids. All goblins seem to be of the same sex, though in truth, this is something of a conjecture. Smaller goblins, perhaps immature ones, are seen among their swarms, but never is any parental nurturing or concern offered them by any of their fellows.
It's difficult to guess the intelligence of goblins. There's no questioning their cunning, but they don't structures or make tools, behaving only as brute beasts. This may be more preference than lack of capacity, as there are reports of them taking up knives and even smallswords and brandishing them in deadly mockery of men. Though they may wear rags or stolen bits of clothing or armor as rude decoration, they are just as happy to go naked.
When swarms of goblins pour forth from the underground, they tend to move toward human habitations, though wild animals will sometimes suffer their assaults. While popular entertainments have made much of the mischievous nature of goblin attacks--their crude pranks, surprise scares, and harassment of livestock--their deadliness is not be discounted. Typically, the actions of the swarm escalate from behaviors which create fear or annoyance to outright attacks with their sharp teeth, stolen weapons, fire, and large numbers. They have been known to consume humans they kill, but that seems to be an after-though.
The infestations may last as little as a night or two or as long as a month, depending on the amount of resistance they encounter. If the swarm doesn't end of its on accord, it can be dispersed by killing a quarter or more of its number.
Scholars have attempted to discern how goblin plagues may be predicted. Folklore suggests that they are "summoned"--perhaps by children entering puberty. Adolescents suffering from the anxiety of an unwanted betrothal, the birth of a new sibling, or other sorts of emotional duress are thought to become unwitting "Goblin Kings" or "Goblin Queens," and call forth their subjects in some psychic manner. Naturalists remained unconvinced but are at a loss to explain the tales of goblins paying rude homage (in imitation of human courtly deference) to a single child in a decimated village or attempting to abduct such a child without harming them in any other way.