If I cast a permanent with x in it's cmc successfully, does x for that permant remain what you declared it to be, or does x become {0} while it remains on the Battlefield?
For example. I cast {mistcutter hydra} where x = {4}, does {mistcutter hydra}'s total cmc become {5} while it is a permanent on the battlefield, or {1}?
Anywhere but on the stack, X always is 0. Even on the battlefield, Mistcutter's CMC is just 1.