Volrath's Motion Sensor 
Enchantment — Aura
Enchant player As this Aura enters, choose one of enchanted player's hands. That player balances this Aura on the back of that hand. When this Aura falls off the chosen hand, sacrifice this Aura and enchanted player loses 3 life.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyNOT LEGALvintageNOT LEGALcommanderNOT LEGAL