Diala has Shattering Blow which you declare an attack, you may discard 1 Command card from your hand to choose and remove 1 die from the target’s defense pool. If the defender is the Royal Champion and he has 1 white and 1 black in his defensive pool, who picks the die?

Sorry if this has been asked.

Diala's controller

Why Diala and not the one being targeted?

Because Diala's action removes the dice, so she selects which one to remove.

The card reads "When you declare an attack, you may discard 1 Command card from your hand to choose and remove 1 die from the target's defense pool." If the designers intended the defender to have the option, they would have used different wording. In Gideon's "Hammer and Anvil" hero class card, we see it state: "The target figure chooses the order in which these attacks resolve." Something like this would have been added to Diala's card if that was the intention.

Thank you