I mean, I can't remember how most games use swarms, this is just my default Swarm Rules plus some body horror stuff.
The gnawing sounds intensifies as you kick the door down, raising tomb-dust. Flickering torchlight reveals a glistening surface. You approach, the noise fills you. You feel a horrible recognition in the pulse of the sound. The Flesh-Hive is visible now, a tumour clinging to the wall, seemingly trying to tear itself apart. The noise stops. The Hive stops. One step closer. The noise crashes back and the Hive disgorges its brood. You are torn apart in seconds, recycled to grow the Hive. Some echo of your memory is forever enshrined in the mass.
The larger the Hive, the higher the HD. The Hive itself is immobile and has no defences, bar it's brood. It has HD of Brood equal to twice it's HD, and can choose to seperate these into many smaller swarms if needed. They are controlled by the Hive through pheromone instructions - outside a radius (HD X 100 ft) the Swarm loses cohesion. The Hive has brood scattered throughout an area equal to HD x 50 ft, and is aware of everything that happens in this area.
HD - Variable
AC - As Leather +1 - it is hard to hit a swarm.
Attack - If you are engulfed in the swarm (swarm can engulf HD/4 rounded up, min of 1) you take (HD)d4 damage as they tear and rip - this damage pool is distributed between all people engulfed as the Hive wishes. The Hive will grow by the number of HD lost this way.