Rumie, Harbinger of Decay

At the beginning of your upkeep, each
creature an opponent controls gets a
spore counter.
Creatures with spore counters get 0/-1 for
each of those counters.
When a creature with a spore counter dies,
create a Mushroom artifact token with
"{1}{t}: Sacrifice this token. You gain 1 life."
Sacrifice X mushroom tokens: Return a creature
card with converted mana cost X from
a graveyard under your control.