Source: Libris Mortis: The Book of the Dead
You are not immediately destroyed when your hit points fall to 0 or lower.
Prerequisite: Cha 13, supernatural ability to drain or damage an ability score or drain energy.
Benefit: Each day that you slake your hunger by draining or damaging a living creature's ability score, or draining a living creature's life force, you gain a necrotic reserve. On days when you have created a reserve, you are weakened but not destroyed when you are dealt enough damage to reduce your hit points to 0 or lower.
A weakened undead acting on the strength of its necrotic reserve may take a single move action or standard action each round (but not both, nor can it take full-round actions). It moves at half speed. Taking move actions doesn't risk further injury, but performing any standard action or any other action the DM deems strenuous, including some free actions such as casting a quickened spell) immediately expends the necrotic reserve. Unless the action increased the weakened undead's hit points, it is destroyed. A weakened undead acting on the strength of its necrotic reserve is also destroyed if it is dealt additional damage after the attack that first weakened it.
An undead may only rely on a necrotic reserve up to once per day, even if it engages in additional feeding following its successful return to positive hit points.
Normal: Undead reduced to 0 hit points or lower are immediately destroyed.