Parasitic Sliver

Every creature your opponents control get
-2/-2 and all slivers you control get +2/+2
for each creature your opponents control affected this way.
{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}{b}: If
combat damage would be dealt this turn to
you or creatures you control, prevent it all
and deal that much damage to any target.