MAGIC: THE GATHERING
You may exile a black card from your hand rather than pay this spell’s mana cost.
Target player reveals their hand. You choose a nonland card from it. That player discards that card.
Any time a spell costs zero mana to cast it has the potential to be very powerful. Unmask is the discard spell of choice for Vintage Dredge and a number of other combo decks. When you don't have to spend mana to protect your combo, you get to play it earlier in the game.