Black Widow, Covert Operative

Stealth Tactics — Whenever Black Widow attacks, target opponent reveals their hand.
You may exile a nonland card from it. Until
the end of your next turn, you may cast that
card and spend mana as though it were mana
of any color to cast it.

Whenever you cast a spell from exile, target opponent must either sacrifice a creature, or Black Widow deals damage equal to the mana value of the card being cast from exile.