Must be played (as well as other "as the action is announced" action modifiers) before action modifiers that are not explicitly played "as the action is announced" (e.g., Conditioning).
101286
Nightshades
Only usable as the action is announced. Choose an ally or younger vampire. The chosen minion gets -1 intercept.
As above, but usable if a minion attempts to block.
Rulings
Replace the action card (if any) before playing it.