Swarms
Swarms are not a single creature, but a mass of smaller ones acting in unison. Individually insignificant, they become dangerous through sheer numbers, overwhelming anything caught within their reach.
Unlike most threats, swarms do not think or plan. They move as a tide, surging, consuming, and dispersing without regard for tactics or self-preservation.
Encounters with swarms are less about defeating a foe, and more about surviving the onslaught.