Jahna, the Reanimator

Whenever a creature token you control dies, you
gain an experience counter.
At the beginning of your upkeep, you may cast a creature spell from an opponent's graveyard
without paying its mana cost if its mana value is
equal to the number of experience counters you
have.
If you have more experience counters than the
highest mana value of all creatures in all
graveyards, creature spells cost {1} less to cast for
each experience counter you have.