Die Hard is a Fallout Tactics and Lionheart perk and a Fallout: Brotherhood of Steel skill (skills in this game work like perks do in other Fallout games).
You don't give up easily, when your hit points get below 20% you get an additional 10% to Damage Resistance.
Davidson in Van Buren had this perk. Combined with his armor and his other perks, like Toughness, Davidson became quite powerful.
Fallout: Brotherhood of Steel
Each level of this skill increases your health for +20 (up to a maximum +200) hit points.
- 1 Rank : 4 Points (+20 Hit points)
- 2 Rank : 8 Points (+50 Hit points)
- 3 Rank : 16 Points (+90 Hit points)
- 4 Rank : 32 Points (+140 Hit points)
- 5 Rank : 64 Points (+200 Hit points)
When your hit points get below 20%, you gain +20% to all resistances and your Armor Class goes up by 5.