Swarm. A swarm is immune to all attacks except for area attacks. When it attacks, it simply moves into the target's square. Any creature which starts its turn in or enters the swarm's square suffers 2d6 damage automatically. Swarms can move through gaps of any size.
Variable size. Swarms can be of any size. The stats above are for a medium-sized swarm. For each size category larger, the swarm gains +1d6 to damage (i.e. an enormous-sized swarm does 3d6 damage), and +10 HEALTH.