Nolva, Puppet Artiste

Wither (Nolva deals damage to creatures in the
form of -1/-1 counters.)
{t}, Put a -1/-1 counter on each other creature
you control: Nolva, Puppet Artiste deals X
damage to each creature you don't control,
where X is equal to the number of creatures
with -1/-1 counters on them.