Plague of Treachery

When Plague of Treachery enters the battlefield,
you may put a -1/-1 counter on target creature.
Spells you cast with Blight, Plague, or Pox in
their name cost {b} less to cast.
Little did the residents of the defiled town of Arkthom
know that the cause of their agony and despair lay
just beneath the surface - in the supposed "solution" to
their harrowing problem.