Pact Weapon is a mythic Artifact — Equipment card from Battle for Baldur's Gate Promos (PCLB), collector number 139s. Current market price: — (foil $26.23), TCGplayer market prices via Scryfall.
As long as this Equipment is attached to a creature, you don't lose the game for having 0 or less life. Whenever equipped creature attacks, draw a card and reveal it. The creature gets +X/+X until end of turn and you lose X life, where X is that card's mana value. Equip—Discard a card.