Level drain is an
insidious and highly
sinister attack form usually associated with
undead monsters. In
AD&D, level drain is often referred to as "
energy drain". Most of the more powerful
undead (in
AD&D, not
NetHack) can drain levels because of their strong connection to the
Negative Material Plane -- low-level
undead such as
zombies and
ghouls exist primarily on the
Prime Material Plane and thus do not usually possess this
ability. I believe the weakest type of
undead which posseses level drain is the
wight, but this depends on your
DM.
The net result of being hit with a level drain attack is that you lose one of your hard-earned experience levels. In NetHack, depending on the type of level drain attack, it is sometimes only necessary to make a single kill to regain the lost level. In AD&D, it is always a major pain to regain a lost level -- spells like heal, atonement, or wish might help.
Certain very evil weapons possess level drain ability, including the artifact weapon Stormbringer in NetHack, Kura-the-Darkness in AD&D, and Uberfetus's Patented Node-Vertising Pimp Stick. Additionally, Stormbringer confers resistance to level drain attacks when it is wielded. Sadly, in AD&D, magical items that give such resistance are few and far between.