Qyburn, Forbidden Alchemist

At the beginning of your upkeep, draw a card
then discard a card. If a creature card is
discarded this way, exile it with X study counters
on it, where X is it's converted mana cost and
put a -1/-1 counter on Qyburn.
Whenever you draw a card, you may remove a
study counter from a card in exile. When the last
study counter is removed from a card in exile,
you may cast it without paying it's mana cost.