Die Hard (Fallout: Brotherhood of Steel)

Die Hard is a Fallout Tactics and Lionheart Perk. So far it hasn't appeared in any other game.
 * You don't give up easily. When your hit points get below 20% you get an additional 10% to damage resistances. (Fallout Tactics)
 * When your hit points get below 20%, you gain +20% to all resistances and your armor class goes up by 5. (Lionheart)