Fortune Thief

{4}{R}Creature — Human Rogue0/1

Damage that would reduce your life total to less than 1 reduces it to 1 instead.

Morph {R}{R} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)

If your life total is less than 1 but you've somehow avoided losing the game, Fortune Thief's effect won't apply at all if you're dealt damage. (2018-03-16)
Fortune Thief's effect applies only if your life total is being reduced by damage. Other effects or costs (such as losing or paying life) can reduce your life total below 1 as normal. (2018-03-16)
Fortune Thief's effect is not a prevention effect. It stops unpreventable damage from reducing your life total below 1. (2018-03-16)
Fortune Thief doesn't change how much damage is dealt; it just changes how much life that damage makes you lose. Abilities such as lifelink will see the full amount of damage being dealt. (2018-03-16)
Fortune Thief won't prevent you from losing the game if your life total becomes 0 or less or if some other effect causes you to lose the game. (2018-03-16)