Swarms
Swarms are seething masses of small creatures, such as rats, snakes or insects, summoned to the battlefield by magical means and set loose upon the enemy. Individually, the critters in a swarm are little threat, but their sheer weight of numbers more than compensates for their size.
Swarms are made up of a number of bases. Each base includes many creatures, but counts as a single model. Other models can fire through them at targets behind them with no To Hit penalty. However, any Swarm base that is hit by a template attack suffer D6 hits rather than 1.