Walking Charnel Pit

({t}: Add {b}.) (Walking Charnel Pit is black.)
When you play Walking Charnel Pit, you may
exile three creature cards from your graveyard.
If you do, Walking Charnel Pit enters the battlefield with a +1/+1 counter on it.
{t}, Exile a creature card from your graveyard:
Put a +1/+1 counter on Walking Charnel Pit.