TomeNET

(5.14) Invisibility                                                             
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Invisibility on monsters is binary, either they are invisible to a player (if
he doesn't have 'see invisible' ability) or they aren't (if he does have it).
Note that ESP overrides invisibility, so if an invisible monster isn't
empty-minded (or has special ESP-cloaking such as 'telepath' ego monsters) you
can spot it with the matching ESP even if you don't have 'see invisible'
ability. Some type of magic may detect invisible monster too, while some other
creature-detection type of magic actually fails against invisible monsters.
Players who don't have 'see invisible' but have infra-vision (and aren't
blinded) can also see invisible monsters if they aren't cold-blooded.

For players being invisible it depends:
-Invisibility from spells has a certain power shown in the spell's info.
-Invisibility from items uses 4/5 of the player's level for power
 (player level is capped at 50).
-Invisiblity from monster form (mimicry users) is similar to invisibility from
 items, except that its power is the average of monsterlevel/2 and playerlevel,
 with monsterlevel capping at 100 and playerlevel capping at 50.
Invisibility powers from different sources do not stack, the best one overrides
the weaker ones.
Some items that grant invisibility also have +mods on them (for example '+3'),
these don't affect the invisibility itself, but are usually just an extra
+stealth bonus or some completely different kind of bonus that item also
happens to grant (use *Identify* to find out).

The actual effect invisibility has on gameplay will also be affected by player
and monster level, the ability to see invisible, and shadow cloaks, as follows
here:

A player who is invisible is harder to spot for monsters in the way that they
only have a certain chance to see the player on each turn, making them move
erratically instead of directly targetting the player. This also helps against
casters, archers or breathers who won't be able to target you as well (often)
as they usually could. Your invisibility power is continously compared to the
monster's level, causing an internal 'dice roll' that determines whether or not
the monster will see you on each turn.

Being invisible costs your body a lot of effort, resulting in getting hungry
quickly.

Invisibility won't work at all on monsters that are..
        -invisible too.
        -game bosses (Sauron, Morgoth, Zu-Aon).
        -Dragonriders, they have ESP.
        -nonliving monsters with empty mind (not using optical senses).
        -able to see through the rogue-specific 'cloaking' ability, these can
         see through invisibility too (compare (0.2) 'cloaking mode'):
         Eyes, angels, Nazgul, nether guards, and unique monsters which are
         both smart and powerful at once.

All other monsters are randomly affected by the player's invisibility,
meaning that they will only sometimes be able to spot him (they will see the
player kind of 'flickering').
The closer a monster is, the better its chances to spot the player.
Some monsters have better chances to spot the player, depending on
        -distance to the player,
        -level of the player (higher level = harder to spot),
        -monster class (Dragons, demons, undead, animals, never sleeping
                        monsters and smart monsters are better at it;
                        orcs, trolls, stupid monsters are worse),
        -the vision power of the monster,
        -the level of the monster,
        -player carrying a light source (not affected by vampires' "intrinsic
         light"), doubles the chance of spotting him,
        -player wearing an aggravating item, further triples the chance.

Invisibility also affects other players:
If another player is invisible and of higher level than you..
        -you can see him if you can see invisible and he's not wearing a
         shadow cloak;
        -otherwise he'll be completely invisible to you.
If another player is invisible and not of higher level than you..
        -you can see him easily if you can see invisible and he's not wearing
         a shadow cloak;
        -you see him 'flicker' if you lack seeing invisible or if he's wearing
         a shadow cloak (or both). He will fade in and out of your sight.
         If you want to look at his equipment with the  '  key, then you'll
         need some timing, might take several tries.
Invisibility (on players) overrides ESP. So it does help in a (blood bond)
duel even if the opponent has ESP.

The effect of invisibility is greatly reduced in pvp (you cannot become
completely invisible even if your level is higher than the other player) but
still gives a slight edge.


Cloaking:
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The ability to cloak themselves is exclusive to rogues and automatically
acquired at level 15. This will effectively render them invisible, but it is in
fact NOT the "Invisibility" that has been described above.
Cloaking overrides any detection mechanism, a cloaked player can only be seen
by party members, certain special monsters, or while the cloaking is weakened
because the cloaked player performed certain (noisy) actions.
See (0.2) for more information about cloaking.