# Invisibility
Invisibility is a property that prevents you from being seen by monsters that lack the see invisible property. Monsters with the invisibility property cannot be seen normally by a player without see invisible, though invisible monsters can be detected through other methods.

Contents

1 Effects

1.1 Effects on other objects and properties

2 Sources
3 Strategy

3.1 Fighting while invisible
3.2 Fighting invisible monsters
3.3 Invisibility and shops

4 Variants

4.1 SLASH'EM

4.1.1 Invisible items

5 References

Effects
While invisible, the monster's glyph cannot be seen on the map unless the monster can be seen through another method, such as telepathy. If you bump into an invisible monster, their square will be marked with an I, and searching will also mark their locations this way; however, this is done before the monster takes action, meaning they will often move to another square afterward. Bumping into a peaceful invisible monster will anger it.
While you are invisible, monsters will have difficulty locating you, and attacking monsters take a -2 to-hit penalty; such monsters will attack the square they think you are located on. Invisibility has no effect on combat between monsters.
An invisible player will be prevented from entering shops by the shopkeeper. ("Invisible customers are not welcome!") The shopkeeper will also prevent invisible players from leaving.

Effects on other objects and properties
Invisibility prevents displacement from working on monsters that do not see invisible. You also cannot scare monsters by applying a mirror unless they can see invisible; similarly, mirrors and sources of reflection will not reflect Medusa's petrifying gaze while you are invisible.
While invisible, wearing a mummy wrapping will render you visible until it is taken off.

Sources
There are multiple methods of becoming invisible:

You can become invisible by quaffing a potion of invisibility, zapping a wand of make invisible at yourself, or wearing a cloak of invisibility; monsters can also do the same, and the potion effect is always permanent for monsters. You can also wear (or eat) a ring of invisibility, which will auto-identify itself when you put it on unless you have invisibility from another source. Except for non-blessed potions, these items will make you invisible either permanently or until you remove them.
The invisibility spell and non-blessed potions of invisibility will grant you temporary invisibility for 31-45 (more) turns.
The Orb of Detection, the Archeologist quest artifact, can be #invoked to toggle invisibility on or off if you do not already have the property.
Eating stalker meat will stun you and turn you invisible for 50-149 (more) turns; if you are already invisible through other means, you will instead gain permanent invisibility and the see invisible intrinsic. Pets cannot gain invisibility from eating stalkers.
Monsters will make use of the wand and the potion, which is always permanent for them, and the disappear monster mage spell renders its caster invisible. Tame and peaceful monsters will only use these if you can currently see invisible. Stalkers and black lights are always generated invisible.

Strategy
Invisibility has a number of perks for especially crafty players. Pets gain no benefits from being invisible, as it does not affect monster-versus-monster combat, and will only make them harder to keep track of.
Tame and peaceful monsters that have a way to turn invisible, including spellcasters, only do so if you have see invisible; this is particularly dangerous if you got the property from drinking a non-blessed potion of see invisible, as you will not be able to see them once it expires. Bumping into invisible monsters will anger them - this is potentially fatal in the case of the watch and shopkeepers.

Fighting while invisible
Invisibility is a good evasive aid, usually resulting in hostile attackers missing more often unless you stay in one place; it is especially useful in the early game, where few monsters will have see invisible, and can aid in escape from unexpectedly powerful foes. This can also double as an offensive aid that helps you keep targets off-balance while picking them off from a distance, e.g., with projectiles or polearms. As the game progresses, you will encounter more monsters with see invisible, limiting its effectiveness (unless paired with something like displacement).
Since its passive gaze relies on you seeing it, a floating eye is not affected by a player's invisibility; however, turning the eye invisible instead will render the gaze ineffective (unless you have see invisible), rendering it harmless and allowing you to attack it with impunity. Invisible floating eyes will also not paralyze pets that cannot see invisible; this is especially useful for pacifists.
Invisibility can actually be a disadvantage in certain scenarios: unicorns in particular cannot see invisible and will not know the player's location to keep a knight's jump away, meaning that hostile unicorns will close in unexpectedly and likely skewer an early character. Conversely, invisible players can safely approach co-aligned unicorns to throw their gems at, and cross-aligned ones will be easier to kill or pacify for a prepared character. Invisibility can also make controlling other monsters for various purposes difficult if they lack see invisible, e.g., luring soldiers into Castle trap doors or luring wraiths away from non-graveyard levels. 
The mummy wrapping is a good cloak for players who elects to remain invisible long-term, but find it necessary to become temporarily visible for certain tasks. Like any other piece of armor, be sure to verify the beatitude of a wrapping before trying it on. If you temporarily remove another cloak to use a wrapping, make sure to replace the cloak once you're done.

Fighting invisible monsters
Invisible monsters can be annoying to deal with, and are especially aggravating if the base monster was already decently strong (e.g., a stalker). Searching can reveal their approximate location and help you take the fight to them, and you can also lure the monster down a narrow hallway to make it easier to target; monsters that drink cursed potions of invisibility will also "aggravate" you, revealing their location to you temporarily and allowing you to plan accordingly.
Pets are not deterred by invisibility and can pick a monster off as it approaches; if the monster is a black light, however, it will likely explode and confuse them, causing them to potentially attack you in turn. Shapeshifters that turn into powerful enough spellcasting monsters can cast disappear, and will retain their invisibility even after they shift forms again.
Monsters can generate with a wand of make invisible as a miscellaneous item; if you recently heard a zap prior to killing an invisible monster, and the monster drops a wand that makes engravings vanish, it is most likely a wand of make invisible.

Invisibility and shops
One of the main disadvantages to invisibility is the hassle of shopping - you will be barred from entering or leaving unless you make yourself visible, which is especially troublesome if the source is a cursed item or gave invisibility that was permanent. Most players should keep a mummy wrapping or two on hand in case they accidentally wear such a cursed item and cannot remove it, obtain invisibility that cannot be toggled, or else simply plan to shop while invisible; they can also try to teleport into and out of the shop.

Variants
SLASH'EM
In SLASH'EM

Invisible items
Main article: Invisible item (SLASH'EM)
Items can also be invisible as well - this includes the corpses of killed invisible monsters, but not any statues created from stoning them. Zapping an item(s) with a wand of make invisible will turn it invisible, as will dipping them in a potion of invisibility; dipping the item in a potion of see invisible or cancelling it will render it visible again. There is a 1⁄1250 chance that a randomly generated item will be created invisible.[1]
The wand of wishing in the Castle is always invisible, but will always be found within its chest under normal circumstances. Mummy wrappings and zorkmids can never be made invisible, and Sokoban boulders cannot generate invisible.[2]
Invisible items mostly function the same as their visible counterparts, with some exceptions:

An invisible blindfold or towel will still blind you if worn.
An invisible mirror will not function when applied unless the monster has see invisible.
Invisible scrolls and books cannot be read if you cannot see invisible.
An invisible weapon has a +3 to-hit bonus against monsters that cannot see invisible.
Invisible items cannot be stolen by a monster that cannot see invisible, with the exception of invisible body armor or a cloak worn on top of a visible t-shirt or body armor; no monster with a theft attack can normally see invisible.[3]
The disarm technique cannot be used if you cannot see the target's weapon.
Monsters will not pick up invisible objects if they cannot see invisible;[4] however, this does not prevent them from moving to the item, with the result that they stand on top of it trying and failing to pick it up repeatedly.

References

↑ mkobj.c in SLASH'EM 0.0.7E7F2, line 683

↑ obj.h in SLASH'EM 0.0.7E7F2, line 145

↑ steal.c in SLASH'EM 0.0.7E7F2, line 277

↑ mon.c in SLASH'EM 0.0.7E7F2, line 1025