Wizkids Rules!

Your opponent chooses one non-villain character die. All other non-villain character dice cannot block this turn.

Does the opponent choose the non-villain die when the action is played or do they wait until blockers are declared?
If chosen immediately, what happens if the opponent makes a different non-villain character die a villain? Can this newly annointed villain block or does the text of Villainous Pact apply immediately to prevent all other non-villains from blocking regardless of later effects?

Example, Ebony Maw

Global: Pay Fist Mask. Target character gains the villain affiliation until the end of the turn.

The opponent would choose one non-Villains Character die when the Action die ability is used.

Since this ability states “cannot block this turn”, it means the second sentence is a persistent effect. Therefore, if a Character die gains the Villains affiliation (e.g. via the Ebony May global) before the Declare Blockers portion of the Attack Step, that die can now block.