Just Lucky I'm Alive
You've had lots of close calls. Whenever you finish a fight with less than 25% health, your Luck temporarily increases by +4 for 3 minutes. The perk also permanently increases your own critical hit damage +50%, as well as grants permanent immunity to critical hits that you would otherwise receive. Obtaining this perk requires that you have neutral Karma, but bonuses apply regardless of Karma or health percentage.
This perk requires you to have all four Fallout: New Vegas story add-ons, as each add-on increases the level cap by 5.