Weeping angels are monsters originally added to UnNetHack, that appear in variants UnNetHack, dNetHack, Slash'EM Extended, and SLASHTHEM. UnNetHack also includes a more powerful form, the weeping archangel.
Weeping angels are inspired by a race of creatures from the BBC Television series Doctor Who, which disguise themselves as inanimate statues in order to prey on the energy of other living creatures. A common method of feeding involves sending their prey into the past. To keep up their disguise, weeping angels in Doctor Who do not move while observed, but can move rapidly if not observed.
The weeping angel's method of feeding, and avoidance of moving when observed, have been incorporated into its NetHack counterparts in various ways.
In UnNetHack, weeping angels have attacks that drain spellcasting energy, drain experience levels, and teleport the victim to another dungeon level. All these attacks are affected by magic cancellation.
Weeping angels's gaze attack continues even locked in place. It causes d4 damage and exercises intelligence (the weeping angel's "reflection in your mind weakens you"), but is completely nullified by reflection (the weeping angel "is covering its face").
Like the erinys and Nazgul, the number of weeping archangels that can be generated is very small. They become extinct after seven have been generated. This is a likely reference to the seven archangels in some Christian traditions.
Weeping angels appear in Slash'EM Extended as well. Their level-teleporting attack has been replaced by a cold attack in that variant.
In dNethack, weeping angels won't move while the player character can see them, but move exceedingly fast when unobserved. Despite their fast speed, they make at most one attack per game turn. While they are immobilized from observation, their AC is greatly amplified. They will stay in rooms and not venture into corridors, to avoid forcing the player into a permanent standoff in a dead-end corridor.
Their angel's touch attack teleports the character to a higher dungeon level, and drains experience levels based on how many dungeon levels up the character is moved. The angel's touch attack can be blocked by both teleportation control and life drain resistance.
Weeping angels are dangerous to look at, however. Viewing a weeping angel may inflict weeping angel contamination, in which the remembered image of a weeping angel begins taking over the characters mind, eventually converting them into a new instance of the weeping angel. Characters contaminated by a weeping angel gradually loose Wisdom to abuse. Once Wisdom goes below 3, the character dies and rises from the grave as a new weeping angel.
During the endgame, weeping angels will enter corridors, and their touch attack no longer teleports characters to higher dungeon levels, and is no longer resisted by teleport control.
Lonely assassins they used to be called. No one quite knows
where they came from, but they're as old as the universe. Or
very nearly, and they have survived this long because they
have the most perfect defense system ever evolved. They're
quantum locked. They don't exist when they're being observed,
in the moment they are seen by any other living creature
they freeze into rock. No choice, it's a fact that their
biology, in the sight of any other living thing they literally
turn to stone. And you can't kill a stone. 'Course a stone
can't kill you either, but then you turn your head away.
Then you blink. And oh yes it can.
"This whole book, it's a warning about the weeping angels, so
why no pictures? Why not show us what to look out for?"
"There was a bit about images. What was that?"
"Yes. Hang on.... 'That which holds the image of an angel
becomes itself an angel.'"