When The Reaper enters, put a -1/-1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on each of up to two target creatures.
Whenever a creature an opponent controls with a -1/-1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on it
diesIs put into a graveyard from the battlefield. (CR 700.4), you may put that card onto the battlefield under your control. Do this only once each turn.