Level drain

Level drain is a status effect that removes a number of levels from a creature. Though not inherently permanent, this effect is always configured to be permanent until cured. 
With the loss of levels comes the loss of most things that progress with levels, including Hit points, Attacks per round, THAC0, Saving throws, Thieving Abilities and spell slots.Spent Weapon Proficiency points, Backstab modifier and learned spells and abilities remain untouched.
In addition, any passive modifiers of the following stats coming from equipment or permanent effectsExamples:- The +2 saving throw bonus from the trial of greed in Hell- A Mage's high level abilities granting extra spell slots are suppressedIESDP: All effects using Timing Mode 2 (equipped) that alter the above stats are suppressed. until the original level is restored:
*Saving throws
*Spell slots
*THAC0, except for the weapon's natural modifier
*Thieving abilities
Temporary modifiers such as those that come from spells will also be suppressed if applied before Level drain, but they can be successfully reapplied over it.
==Gameplay==
*A creature drained down below level 1 explodes and dies permanently.
*Dying from reaching level ≤0 may destroy non-critical loot, like with the Disintegrate and Petrification effects or with overkill from Cold damage. Disabling Gore in game options will avoid this.
*A Simulacrum, whose lowered level is implemented by a type of Level drain that ignores immunities, may have significantly worse saving throws than its creator. It is advised to consider boosting it with spells if possible, to compensate for the loss from equipment and permanent passive bonus.
*Multi-class characters will be affected on all of their classes (e.g. a Fighter / Mage / Cleric drained by one level will lose one fighter, one mage, and one cleric level).
*Dual-classed characters will only be affected by Level drain on their active class. This can cause their first class to become inactive again if their second class level drops below their first.
*Experience can still be acquired even though the character sheet does not display it, but levelling up is impossible.
*A sufficiently level drained creature can be subjected to spells that wouldn't otherwise affect it such as Death Spell, Sleep and other spell afflictions that no longer work after a certain level threshold.
Note: Level drain must not be confused with ability score drain, which relies on other status effects such as Intelligence modifier by the Mind Flayer attack, or strength attribute drain from a Wraith Spider, etc. Things that cure or make a creature immune to Level drain don't work on ability score drain unless they also target that. For example, Greater Restoration should work in most cases but not Lesser Restoration.
==Level-dependent spells and abilities==
As noted in the gameplay section, level drain effects can be used to put enemies in range of spells and abilities whose effects take a creature's level/hit dice into consideration. Those spells and abilities are:
*Aura of Despair (at Blackguard levels 15-19 enemies under level 9 suffer Panic, no save, at Blackguard levels 20+ enemies under level 19 suffer panic, no save)
*Chaos (enemies under level 5 get no save)
*Cloudkill (targets ranging from levels 1-4 are slain and those of levels 5-6 save vs poison at -4 or are slain as well)
*Color Spray (targets under level 5 are the only ones affected by this spell)
*Command (enemies under level 6 get no save)
*Deathblow & Greater Deathblow (enemies under level 11/under level 13 are slain, no save)
*Death Spell (enemies under level 9 are slain, no save)
*Holy Word (Undead and evil enemies ranging from levels 1-3 are slain, those of levels 4-7 are stunned for 1 turn, those of levels 8-11 are slowed for 1 turn and get 75% spell failure, and those of levels 12+ are deafened for 1 turn, no save against any of these effects)
*Prismatic Spray (targets under level 8 are blinded for 5 rounds, no save)
*Sleep (targets under level 5 are the only ones affected by this spell)
*Storm of Vengeance (enemies under level 9 are slain, no save)
== Prevention ==
Besides the methods in the info box on the right (Negative Plane Protection etc.) which flat-out make you immune to level drain directly, as with all effects it can also be prevented by not being hit with the source of the effect. For example, Vampires' level drain is tied to their natural weapons and thus can be avoided by Protection From Magical Weapons or high Armor Class, whereas level drain from spells like Energy Drain can be prevented by getting protection against the spell (Spell Immunity, Shield of the Archons, Spell Trap etc.).