An invisible barrier holds back vermin. A vermin with less
than one-third the character's level in HD cannot penetrate the
barrier. A vermin with at least one-third the character's level
in HD can penetrate the barrier if it succeeds at a Will save.
Even so, crossing the barrier deals the vermin 2d6 points of
damage, and pressing against the barrier causes pain, which
deters less aggressive vermin.