Kozilek, Reality's True Form

When you cast Kozilek, Reality's True Form, add {c}. For the rest of the game unused mana doesn't empty from your mana pool.
At the beginning of each upkeep, double the amount of {c} in your mana pool.
Whenever a player casts a spell for the first time each turn, you may pay {x}, where X is it's converted mana cost. If you do, counter or change the controller of that spell.