Death, the Persistent End

Death, the Persistent End may only be cast if there is at least one Reaper in the graveyard
When Death, the Persistent End enters the battlefield target creature gains a +1/+1 counter, target creature will be destroyed if Death, the Persistent End leaves the battlefield
Once per turn discard a card or exile Death, the persistent end
If Death, the Persistent End is sent to the grave yard, Death, the Persistent End will return to the battlefield in during next main phase