SEWER RATS

Use this Stratagem just before you set up a
SWARM / INFANTRY unit during deployment.
Instead of setting up that unit on the battlefield,
you can place them to one side and say that they
are infiltrating a sewer network. At the end of
any of your Movement phases, you can set the
unit up anywhere on the battlefield at street level
that is more than 9" from any enemy models and
is not within a ruin or a Sector Mechanicus
structure.