TomeNET

(4.12) Malicious effects monsters can suffer                                    
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Monsters can be affected by similar effects as players.
Those are:

Fear, blindness, confusion, sleep, slowing, and stun.

Note (except for 'stun' effect):
      There are basic spells or magic devices that don't do anything except
      trying to cause one of these effects directly, for example a 'wand of
      slow monsters' or the spell 'scare'.
      It is important to know that unique monsters are always unaffected by
      these basic spells. Other monsters have a chance to resist, depending on
      their level vs the power (level) of the spell.

Effect details:
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Fear is a temporary effect, causing the monster to flee. Many monsters turn
        afraid if their hit points drop very low. Wears off after a while.
        Monsters that have NO_FEAR flag cannot be scared.
Blindness and confusion are currently the same thing, resulting in erratic
        movements and being unable to cast spells or attack. Usually only
        low level monsters are prone to these.
        They are temporary effects that wear off after a while.
        Monsters that have NO_CONF flag cannot be confused.
        Undead, demon, dragons, nonliving monsters and monsters of level 45 or
        higher cannot be blinded.
Sleep is especially nice if it's cast as an area effect. Gets you out of
        trouble, although most higher monsters are immune to it.
        Monsters will wake up after a while, depending on your stealth.
        Sleep effects are NOT cumulative.
        Monsters that have NO_SLEEP flag cannot be slept. Note that they may
        still be asleep when you meet them though.
Slowing on monsters is currently a permanent effect. Very effective, although
        many higher monsters are immune to it. Monsters may resist sometimes so
        you might need to try again.
Stun    (Note that "stunned" and "dazed" are the same thing.)
        Stun can be caused by special martial art attacks among other things.
        Stunned monsters have a harder time to successfully cast spells or hit
        you in melee combat. Also monsters cannot interfere with your actions
        while they are stunned. Many monsters are immune to stun, but also
        many monsters - even of high level - are not.
        Monsters can also be heavily stunned or knocked out, same as players.
        Stun is temporary and will wear off, if no new stun effect is applied.
        Monsters that have NO_STUN flag cannot be stunned.

Monsters have a saving throw, and unique monsters are usually immune to some
of these crowd-control effects.