Zillanith, Gaea's Avenger

Zillanith, Gaea's Avenger enters the battlefield
with a number of +1/+1 counters on it equal to
the amount of mana spent to cast it.
Whenever you cast a spell with {x} in its mana
cost, put {x} +1/+1 counters on this card where
{x} is equal to how much you paid for {x}.
You may play spells with {x} in their mana cost
from graveyard or exile, but you must pay {x} mana costs by removing that many +1/+1
counters from this card.