Ki Ransa, Deathly Undertow

Ki Ransa, Deathly Undertow can't be blocked.
Whenever another Merfolk creature enters the battlefield under your control, creatures your opponents control get -1/-1 until end of turn.
Whenever an opponent casts a creature spell,
you may pay {1}. If you do, create a 1/1 blue and black Merfolk creature token.