Grievous, Sith Engine

Whenever you cast a spell, put a +1/+1
counter on Grevious, Sith Engine for
each of that spell's colors.
Remove five +1/+1 counters from Grevious:
Add {w}{w}{u}{u}{b}{b}{r}{r}{g}{g} to your
mana pool. Activate this ability only once
each turn.