Yasei, Nature's Harmony

{1}: Add two in any combination of {w}, {r}, or {g} to your mana pool. Use this mana only to cast a creature spell. Activate this ability only once each turn.
Whenever you cast a creture spell, you gain life equal to its converted mana cost.
At the begining of each upkeep if you gained 10 or more life last turn, exile Yasei, Harmonious with Nature, then return it to the battlefield transformed under its owner's control.