Draining is an effect that withers its victim, reducing their durability. Its exact effect varies depending on whether the target is a monster or the player character.
Draining can come from certain types of negative energy damage or necromantic effects, from certain divine abilities, from remaining airborne above lethal terrain after your temporary flight has expired, from attempting to evoke an item with the invisibility ego (both success and failure cause draining), or from removing items with the *Drain ego.
When used against susceptible monsters, it inflicts the Drained status effect, causing the creature to function as though it had fewer hit dice than it actually does. This reduces the strength of their attacks and spells as well as their willpower. This status effect can be stacked through multiple hits, but will wear off over time.
Draining effects directed against the player will temporarily reduce their maximum HP. At extreme levels of draining, you could be drained all the way to down 1 HP.
These drained HP will gradually recover as you gain experience, eventually being restored entirely. The worse the drain, the longer it'll take to work off the effect; be especially wary when fighting while heavily drained.
Late-game characters that have had the misfortune to be heavily drained may wish to seek out a relatively non-threatening side branch (such as the Elven Halls) in order to recover some of their health. Of course, gaining more experience in a challenging branch will removing the draining more quickly.
Draining from the brand and spells count as necromancy, and as such can be reduced with negative energy resistance; damage and draining are reduced by 50/75/100%, depending on how many ranks of resistance you have.
Draining from item egos and god effects are not related to negative energy, and cannot be resisted (they will even affect undead players).
- Prior to 0.26, draining temporarily reduced a player's skills instead of their maximum HP.
- Prior to 0.16, negative resistance lowered draining and negative damage by 33/66/100% as your resistance increased.
- Prior to 0.15, draining monsters would permanently reduce their HD, making them slightly less dangerous, reducing their XP value, and eventually killing them outright if they hit 0 HD.
- Prior to 0.13, getting drained would damage your character's experience, potentially forcing you down a level. This was permanent and was immediately lethal if you were ever reduced below 0 XP, but you could simply gain more experience to make up for it. This version of draining had no impact on your skills.