Mjölnir, Hammer of Thor is a mythic Legendary Artifact — Equipment card from Marvel Super Heroes (MSH), collector number 303. Current market price: $24.15 (foil $33.69), TCGplayer market prices via Scryfall.
When Mjölnir enters, it deals 4 damage to up to one target creature. Double all damage equipped creature would deal. Equip worthy {1} (A creature is worthy if it's a legendary non-Villain that's red and/or white.) {2}{R}, Discard this card: It deals 2 damage to each creature.