Undead in NetHack are a group of unliving monsters. They include the following monster classes:
Undead monsters are represented internally by the monster attribute
M2_UNDEAD. Undead take an extra d4 of damage from blessed items. Contact with holy water (including quaffing) also harms them, including the player if polymorphed into an undead form; however, unholy water actually exercises constitution. Undead deal double damage in the hour of midnight.
Reading the Book of the Dead while cursed will summon hostile undead. The blessed book will pacify them, and tame them whenever possible if you are chaotic. Sunsword does double damage against undead monsters.
Undead and garlic
All undead dislike garlic and will avoid it where they can.
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"Further elaborate on their traits"