Invisible

In Crawl, being invisible is treated quite differently based on whether it is the player or the monster that is invisible.

The Invisible Player

You can become invisible by:

• zapping yourself with a wand of invisibility
• drinking a potion of invisibility
• casting the level 6 Hexes spell Invisibility on yourself
• using the ability granted by a ring of invisibility or certain artifacts
• drawing Nemelex Xobeh's Shadow card
• quaffing from a fountain of sparkling water (2.5% chance)
• zapping yourself a wand of random effects (5.5% chance)

The potion and wand are always successful, but the ability's success is based on your Evocations skill. The spell's success, of course, is dependent upon your Hexes and Spellcasting skills and intelligence.

An invisible player gets the following bonuses and penalties:

• -5 to their melee to-hit number if you yourself cannot see invisible. (Official justification: you can't see your weapon.)
• Anyone attacking you in melee incurs a 35% reduction in their to-hit number.
• Any of your melee victims who have shields have their blocking roll divided by three.
• You are more likely to be able to stab your opponents.
• Ranged attacks by opponents who can't see invisible have their to-hit number halved.
• You have a 60% chance of gaining a point of magic contamination when handle_time() is called.
• You can only dissipate magic contamination via "violent discharge" or the wrath of Sif Muna.
• Greatly increased metabolism (+5 to hunger rate).
• Reduced piety for Zin followers.

Invisible Monsters

Invisible monsters are very dangerous to a player who cannot see invisible. If you cannot see invisible, then in melee with invisible monsters:

• Your shield block rolls are divided by three.
• You suffer a -10 penalty to your EV against their melee attacks
• You suffer a -6 penalty to your melee to-hit number against invisible monsters.

Permanently Invisible

These monsters are permanently, inherently invisible:

• unseen horrors
• vapours
• ghost moths
• shadow wraiths
• lorocyprocas

Casters of Invisibility

Some spellcasting monsters can make themselves invisible:

• orc wizards
• ogre mages
• sky beasts (by inherent ability, not a spell)
• deep elf soldiers, deep elf fighters, deep elf knights, deep elf mages, deep elf conjurers
• liches, ancient liches
• vampires, vampire mages, vampire knights
• rakshasas (except Mara and rakshasa clones generated by Rakshasa Summon spell)
• boggarts
• necromancers, wizards
• the uniques Sigmund, Blork the orc, Erolcha, Ilsuiw, Kirke, Erica, Maurice, Boris, Norris, Frederick, The Enchantress, Gloorx Vloq

This effect has a finite duration, but the monster can always recast it after it has run out.

Potion or Wand Users

Monsters who know how to use magic items may use potions or wands of invisibility to make themselves invisible. This effect has a finite duration, but the monster can always reattempt it after it has run out.

Submerged Monsters

Certain monsters can have the inherent ability to render themselves effectively invisible by submerging. However, this is a defensive maneuver only: they are always revealed if they become adjacent to, or if they attempt to attack, the player. None of the conventional methods described below for detecting inherently invisible monsters work on submerged monsters:

• in deep water: crocodiles, baby alligators, alligators, swamp worms, sea snakes, big fish, giant goldfish, jellyfish, sharks, electric eels
• in lava: lava worms, lava fish, lava snakes, salamanders
• in clear tiles: trapdoor spiders, air elementals

Detecting Invisible Monsters

By Item, Spell, Mutation

• a ring, headgear, or weapon artifact with the see invisible ego
• the level 4 Charms spell See Invisible
• the Acute Vision mutation (held inherently by all spriggan, vampire, naga, and felid players), rank 3 of the Antennae mutation, or rank 3 of Jiyva's Eyeballs mutation

Backlighting

Under some circumstances, a target may become backlit. This renders its invisibility ineffective. Backlighting occurs as a result of the level 1 Hexes spell Corona, having a halo (say, from wielding the Mace of Brilliance artifact), being in the halo of a devout worshipper of The Shining One, or as the result of mutagenic glow. Using Sticky Flame on a substantial creature that's not on a water tile is also effective; this method is best attempted in narrow hallways.

Invisible Monsters Revealed as a Disturbance

Certain less stealthy monsters, like orc wizards and sky beasts, will occasionally leave disturbances while invisible, represented in the console game by the { glyph. Additionally, all substantial invisible monsters, when passing through a cloud, are temporarily rendered visible, as the disturbance glyph. Should you examine the disturbance (with the x key), you will find the creature's identity. Thus anything the player can do to create clouds when an invisible creature is near will aid him in fixing that creature's location:

• scrolls of fog
• using Ice Magic in and over lava, creating steam clouds
• using Fire Magic in and over water, creating steam clouds
• firing any persistent cloud-based magic: Poisonous Cloud, Mephitic Cloud, etc.
• casting Ring of Flames
• Fedhas Madash's Rain ability

Also, any (non-levitating) invisible monster in shallow water is always rendered as a disturbance. So, any way you can move the battle to shallow water will aid your locating the unseen monster.

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Commentary

The usefulness of Invisibility for the player is pretty limited.

• The Invisibility spell is a level 6 spell that causes magic contamination whenever you cast it on yourself. This means it is as costly and dangerous as Haste, despite it being completely useless in a significant number of situations and marginal in a lot of others.
• You can't safely get rid of magic contamination while you're invisible.
• If you don't have see invisible, being invisible makes it harder for you to hit your opponents in melee.
• Most of the monsters you'd most like to not be seen by can see or sense invisible.

It is, however, very useful for stabbers. It's possible to wipe out whole colonies of sleeping enemies without waking anyone up thanks to the large stealth bonus.