Aang, Airbending Master

Legendary Creature — Human Avatar Ally

When Aang enters, airbend another target creature. (Exile it. While it's exiled, its owner may cast it for rather than its mana cost.) Whenever one or more creatures you control leave the battlefield without dying, you get an experience counter. At the beginning of your upkeep, create a 1/1 white Ally creature token for each experience counter you have.