Skeletal Bogwalker

When Skeletal Bogwalker enters the battlefield, remove all -1/-1 counters from it and you may have it become a Basic Swamp
Whenever an opponent attacks you, you may sacrifice a random Swamp. If Skeletal Bogwalker is sacrificed this way, destroy target attacking creature . When it enters the battlefield next, it gets x +1/+1 counters, where x is the converted mana cost of all creatures destroyed this way.
Persist