Rad Child (Fallout: New Vegas)

Rad Child is a Fallout: New Vegas, Fallout Tactics and J.E. Sawyer's Fallout Role-Playing Game perk and a Fallout: Brotherhood of Steel skill (skills in this game work like perks do in other Fallout games).

When you get any sort of radiation sickness, your health regenerates, the effect becoming more powerful with every threshold reached. However, radiation will continue to build up in your system, and can still kill you at the fatal threshold as normal.

Fallout: New Vegas
The health regeneration kicks in when the player gains radiation sickness and becomes more powerful depending on the severity:
 * Minor (200 rads): +2 health per second
 * Advanced (400 rads): +4 health per second
 * Critical (600 rads): +6 health per second
 * Deadly (800 rads): +8 health per second

You will still die when you reach 1000 rads. This perk does not heal limbs.

This perk is often overlooked due to the high requirements (70 Survival and radiation sickness), but the effect is extremely potent, even with just the first level of radiation (which carries only a -1 Endurance penalty).

However, the greatest combat benefit for this perk is provided at critical or deadly radiation poisoning. There is a significant SPECIAL penalty (-3 END, -2 AGI, -1 STR for critical; an additional -1 STR for deadly), but with such high regeneration the player becomes virtually immortal to most enemies, simply by out-healing many enemies' damage rates. Exceptions would be high-damage-per-hit creatures like Deathclaws. In fact, the difference between even just minor and advanced Radiation Poisoning can be stark in terms of regenerating damage.

Fallout: Brotherhood of Steel
With this skill, Cain restores HP whenever he takes radiation damage. To take it, the player must be Level 10.