Empyrean Rapture

The condition is only met during your turn, when you’ve put a card with Herald in its name into your soul. It is not met during an opponent’s turn.

The condition must be met before you activate your first hero ability that turn, in order to get the discount. If you’ve activated a hero ability before the condition is met, your next hero ability on the same turn is not discounted, because the effect only applies to the first hero ability you activate during the turn.