First strike
When Nightshade Assassin enters the battlefield, you may reveal X black cards in your hand. If you do, target creature gets -X/-X until end of turn.
Madness {1}{B} (If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)
If the target creature becomes an illegal target, you don’t reveal any cards.
You choose how many cards to reveal and which as Nightshade Assassin’s triggered ability resolves. Players can’t take actions between the time you reveal cards and the time the target creature gets -X/-X.