Kaervek the Merciless 
Legendary Creature — Human Shaman
Whenever an opponent casts a spell, Kaervek deals damage equal to that spell's mana value to any target.
Legality
standardNOT LEGALpioneerNOT LEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL