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As long as Pact Weapon 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.
If an opponent gains control of the creature that Pact Weapon is attached to but doesn't also gain control of Pact Weapon, you still control Pact Weapon and still don't lose the game for having 0 or less life as long as it is attached.
If the card draw is replaced with another effect, the equipped creature won't get any bonus to its power and toughness and you won't lose any life. This is true even if the card draw is replaced by one or more card draws.
Similarly, if an opponent gains control of the equipped creature without gaining control of Pact Weapon and attacks with it, you will draw a card, reveal it, and lose life.
