Warning is a property conveyed by wearing a ring of warning or acquired by certain roles. It gives you advance warning of nearby monsters you can't see by representing them with a number from 1 to 5 based on their level. (1, 2, 3, 4, 5) The number is ML/4, where ML is the monster's level (though this will go no higher than 5, and 0 is not displayed). The glyphs displayed are described as an "unknown creature causing you <feeling>", where <feeling> is one of concern, anxiety, disquiet, alarm, or dread for warning levels one to five respectively. Warning levels will be chosen at random if you are hallucinating.
In graphical user interfaces using the default tileset, such as the Windows client nethackw, the numbers are replaced with colored question marks: . Colors of the question marks correspond to the colors of the numbers given above.
Warning is considered a useful ring in ring strategy.
Besides the ring, some quest artifacts convey warning: the Orb of Fate and the Master Key of Thievery (and in SLASH'EM, The Candle of Eternal Flame and The Storm Whistle) . A few roles also gain intrinsic warning when reaching a certain experience level: XL 7 as a monk and XL 15 as a caveman, healer, priest, or wizard.
The exact criteria for warning to show you monster locations are:
- The monster must not be peaceful.
- You must be under 10 tiles from it (i.e., it must be strictly within a ten-tile circle centered at you).
- Its level must be greater than or equal to 4.
Warning can't tell you exactly which monster you're up against, unlike ESP, however, if you have both and the monster isn't seen through telepathy even though you're near enough (or blind) then you know it must be one of the mindless kind. The following table lists which monsters each warning may represent in this case. Note that a monster's level can vary within some limits, so for example an ochre jelly can merit a warning number 1 or 2, depending on the dungeon level and your experience when it is created:
|Warning symbol||Possible monsters when also telepathic|
|(I, no warning)||bejvvyFFFFFFFMMPZZZZZZZ'''|
Caution: Moving with m while blind will still attack in some cases, possibly petrifying you. This happens whenever the warning mark does not cover an I from a remembered monster position. Likely, this is a bug.
- "You feel sensitive!"
- You gained intrinsic warning through level gain. (Except in SLASH'EM, a Necromancer at level 3 only gains intrinsic warning against undead.)
- "You feel less sensitive!"
- You lost intrinsic warning through level drain.
- "Your Sting glows light blue!"
- You are wielding Sting, and the monster is an orc.
- "Your rings both glow <color>!"
- You are wearing two rings of warning.
- "Your left/right ring glows <color>!"
- You are wearing one ring of warning, on your left/right hand.
- "You feel apprehensive as you sense a <color> flash."
- You have intrinsic warning, or warning from an artifact.
Messages that are higher in the above list took precedence over the ones that are lower.
The color of the flash was determined in the same way as the warning numbers in modern versions: pink corresponds to "1", red to "2", ruby to "3", purple to "4", and black to "5".
When the hero was hallucinating, the colors were replaced by hallucinatory colors, rings were referred to as "mood rings", and the message for intrinsic warning or warning from an artifact was replaced by "Your spider-sense is tingling...", a reference to Spider-Man. No warning messages were displayed when the hero was blind.
No warning messages were displayed if one had been displayed within your last ten actions, unless the new message reflected a higher level of warning than the previous one. Nevertheless, at the time, warning was not popular among NetHack players, who considered it nearly useless at best, and a highly-annoying source of spam messages at worst.