Rat Swarm

Swarms represent many creatures on a single 40mm x 40mm base. This base is treated as a single model with several Wounds and Attacks. A Rat Swann base fights at full effect until it has taken 5 Wounds then it is removed. Rat Swarms are Unbreakable and cannot be joined by characters.

All Skaven units, except other units of Clanrat Slaves, automatically pass Panic tests caused by units of Clanrat Slaves.
All Skaven units automatically pass Panic tests caused by Rat Swarms.

Unit are skirmishers

Rat Swarms don't block line  sight of other units. Note that this does not allow  other skirmishers to pass through them.