Sai, Thought Infiltrator

Flash
Sai, Thought Infiltrator costs X less to cast,
where X is the amount of cards that have entered
the graveyard this turn.
When Sai, Thought Infiltrator enters the battlefield, exile all graveyards. You may cast
cards exiled with Sai, and spend mana as if it
were mana of any color to cast spells this way.