Mystery Key (CLB) from Commander Legends: Battle for Baldur's Gate, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Yeong-Hao Han
• Card Text:
  When equipped creature deals combat damage to a player, sacrifice Mystery Key. If you do, draw three cards.
  Equip 1 (1: Attach to target creature you control. Equip only as a sorcery.)