Aang, Airbending Master {4}{W}

When Aang enters, airbend another target creature. (Exile it. While it's exiled, its owner may cast it for {2} 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.