Improved Toughness( Libris Mortis: The Book of the Dead, p. 27)
You are signifi cantly tougher than normal.
Base Fortitude save bonus +2,
You gain a number of hit points equal to your current Hit Dice. Each time you gain a Hit Die (such as by gaining a level), you gain 1 additional hit point. If you lose a Hit Die (such as by losing a level), you lose 1 hit point permanently