Grave hungry

Whennever a creature enters the battlefield from
the graveyard it's controller must sacrifice a
creture and create a food token, put a grave
counter on that creature
{10}:put target creature from a graveyard on
the battlefield under your control, this ability
costs 1 less to cast for each creature with a grave
counter on the battlefield, any player can activate
this ability, spend only black mana to use this
ability