Mystery Key 

Artifact — Equipment
When equipped creature deals combat damage to a player, sacrifice this Equipment. If you do, draw three cards.
Equip (
: Attach to target creature you control. Equip only as a sorcery.)
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL