The Lorekeeper's Tome

When a Giant Spider's egg-sac bursts, or a nest is disturbed in the deep dark, the result is a Swarm of Spiders: a skittering carpet of hundreds of small bodies, each bite no worse than a pinprick, all of them together a creeping wave of venom that climbs the legs and finds every gap in armor. The horror of it is less the danger than the skin-crawling wrongness, and even hardened delvers have been known to break and run from a floor that has suddenly begun, all at once, to move.