Mobstrosity

A King Mob or Mobstrosity is a human swarm: a regular rabble changed by rare conditions to a strange entity of singular purpose. The people making up the entity never appear to communicate but nevertheless move and act with uncanny coordination. A mobstrosity never forms from less than 20 individuals and seldom more than a 150. It seldom dissipates without blood being spilled.

In game terms a mobstrosity acts like a swarm (and has the usual characteristics of such) except that it's made up of medium sized creatures. Any to-hit roll but a "1" hits a mobstrosity, but it only takes half damage from piercing or stabbing weapons. Attack from the vanguard or outer edge of a mobstrosity does 2d6 points of damage per round. Being engulfed or trampled by it deals 5d6 per round.

Mobstrosities are best defeated not by direct engagement but by removal of the thing that led to its formation. This is often a spell or a cursed item of some sort. When a nidus for its creation can't be found, it's most prudent to get out of the area of the swarm. There are reports of the peculiar madness of a mobstrosity  being infectious and drawing new members into it.

MOBSTROSITY
HD 15+  AC: 10 (see above) Attacks: 1 swarm (see above) Save: F1 (has certain spell immunities due to its nature)