Heart of Darkness (3.5e Invocation)
From D&D Wiki
This invocation infects your heart with a seed of negative energy. You are protected as if by the spell death ward, and negative energy (such as from an inflict spell) heals you rather than damages you. If you are a living creature, positive energy (such as a cure spell) still heals you as well.
In addition, whenever you are under the effects of fast healing, such as when you have activated your fiendish resilience class feature, you heal 1 point in each damaged abiliy score every round, and 1 point in all drained ability scores every hour (if appropriate).