Swarm

Swarms are dense masses of fine, dimunitive, or tiny creatures (usually vermin) that would not be particularly dangerous in small groups, but can be terrible foes when gathered in sufficient numbers. Many different creatures can mass as swarms; bat swarms, rat swarms, centipede swarms, locust swarms, spiderling swarms, and spider swarms for example. A swarm of tiny creatures consists of about three hundred non-flying creatures or one thousand flying creatures. A swarm of diminutive creatures consists of roughly fifteen hundred non-flying creatures or five thousand flying creatures. A swarm of fine creatures consists of perhaps ten thousand creatures flying or not. Swarms of non-flying creatures include many more creatures than could normally fit in a ten foot square area, because creatures in a swarm are packed tightly together and generally crawl over each other and their prey when moving or attacking.

In Warcraft III
The spell Carrion Swarm created a swarm of bats.

In World of Warcraft
The spell Carrion Swarm deals Shadow damage.