Yahenni's Expertise 
Sorcery
All creatures get -3/-3 until end of turn. You may cast a spell with mana value 3 or less from your hand without paying its mana cost.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL