Mjölnir, Storm Hammer is a rare Legendary Artifact — Equipment card from Assassin's Creed (ACR), collector number 170. Current market price: $4.30 (foil $4.63), TCGplayer market prices via Scryfall.
When Mjölnir enters, attach it to target legendary creature you control. Whenever equipped creature attacks, tap target creature defending player controls and put a stun counter on it. Then Mjölnir deals damage to each opponent equal to the number of tapped creatures that opponent controls. Equip {4}