Resource - Elite Attack Die
From: Dominion War OP1 Participation Prize
Use of the Elite Attack Die does not have to be declared until dice are actually rolled. If an Attack is cancelled by an effect like Varel, you may still use your Elite Attack Die on a different attack that round. http://boardgamegeek.com/article/13723031#13723031
The decision to use the die may be made before the player rolls at least one attack die.