Unbreakable

Combat

As a veteran Gray Maiden, you can endure a staggering amount of punishment.

Prerequisite: Con 13, Endurance, Toughness, fighter level 4th.

Benefit: Whenever you gain 1 hit point as a fighter favored class benefit, you gain an additional hit point. Treat your effective Constitution score as being 4 higher for the purpose of determining when you would die from hit point damage. These benefits are retroactive.

Source: Adventurer's Guide