Healing Factor (3.5e Variant Rule)
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This variant rule includes more extensive elaboration regarding one's healing factor, and introduction of rates of healing slower than fast healing but quicker than regular healing, as well as the ability to restore points of ability damage and even drained ability at accelerated rates.
Healing Factor (Ex): Humanoid ozhalig restore 1 HP per hour of activity or 1 HP per 20 minutes of rest, 1 point of ability damage per hour and 1 point of drained ability per day, and have a +12 bonus on Fortitude saves.
This description includes the rate the creature restore hit points and points of ability damage/drained ability and the bonus they gain against toxins and diseases (Fortitude bonus).
Of course, the description assumes the creature has 1 HD. If they have more, then the number of HPs they restore per the noted time is multiplied in the number of HDs they have, just like with ordinary healing.
Additionally, a creature may have both regeneration and fast healing, though mostly the fast healing's rate will be slower. This represents creatures that heal quickly even if wounded by weapons/effects that inflict them lethal damage, however still slower than they heal otherwise. Such creatures are usually very powerful, with very high CR and LA (if they are even playable).
Advanced Healing Factor (Ex): Vampires... have regeneration 10/silver and magic and fast healing 1, and restore 1 point of ability damage per round and 1 point of drained ability per minute.