Artifact chaotic helm Helm of the Underworld, modeled after Hades’ helm. Grants drain resistance, invisibility, magic resistance, and passive detection of buried objects (or phasing, if this is too difficult to implement) when worn. When invoked, it does a stronger version of the chaotic turn undead effect (rebuke or tame undead).
Cursed potions of invisibility remove any permanent intrinsic invisibility you have, but still give you the normal cursed duration of temporary intrinsic invisibility, so after that duration you will become visible again. Thus, they are a controllable way to remove the permanent intrinsic and are also still useful for characters without invisibility yet.
You can pour a potion of oil over yourself to temporarily block any invisibility for 200 turns. While oiled, you take double damage from any fire source, and you can slip through boulders diagonally without worrying about how much stuff you’re carrying.
Add a potion of milk, working along the same lines as milk in Minecraft, which cancels potion effects (or since not many potions have lasting effects that you’d want to cancel, it cancels various temporary good and bad status effects in general, like confusion, stunning, and invisibility, but not nausea since drinking milk usually makes nausea worse in real life.
Getting cancelled should end a bunch of magical effects:
Because it’s a status effect with some downsides, make unicorn horns cure invisibility by removing it. Maybe temporary speed too.
A helm of darkness artifact, inspired by the one from Perseus’ myth; when worn it confers invisibility and produces darkness in a radius around the wearer.
An invisible displacer beast that you can’t see should not be able to use its displacement abilities in combat with you. (If you have see invisible, it should still be able to use them.)
Artifact luckstone Somebody Else’s Problem that confers invisibility and stealth while carried.
If an invisible monster (or the player) is doused in a potion of see invisible from a thrown bottle breaking on them, they become visible again (unless extrinsically invisible which overrides this).
If you are not deaf, non-stealthy monsters that you can’t see (because they’re out of sight or invisible) occasionally show up as an I when they move. Blindness has no effect, except indirectly that adjacent spaces also count as out of sight.
The spell of invisibility has an advanced form which casts a make invisible beam, like the wand does.
Detect monsters shows invisible monsters as I (or whatever S_invisible happens to be).
Stalker corpses don’t confer see invisible anymore, and have their odds of conferring invisibility decreased from 100%.
A sessile invisible monster with a nasty passive attack.
Monster that steals your intrinsic invisibility (but only that, unlike gremlins).
When a monster disappears by becoming invisible (and you can’t see it anymore), place an I on its square.
Invoking artifacts for invisibility makes you visible again if you are already permanently invisible. Also, move the Orb of Detection’s invoke invisibility ability to some lesser artifact because it’s a lame power for a quest artifact.
Seduction (possibly for nymphs as well as foocubi) may not work if the seducer is invisible.
Quaffing cursed invisibility removes the invisibility intrinsic if you had it. Possibly the same for see invisible.
If you’re invisible, peaceful monsters may bump into you and get mad.
Shopkeepers refuse to deal with invisible customers completely.
Blind or eyeless monsters aren’t affected by invisibility or displacement.
Invisible monsters wearing a mummy wrapping show up as an I (you don’t get to see what they actually are), but an I that moves and doesn’t persist on the map.
Invisibility works completely when the invisible monster and the detecting monster are far away. When within 5 (or 4?) squares of each other, its location is known to within 1 space (fuzzing the real location by 1 space in all directions). When the invisible monster takes a direct offensive action, or with some probability when it is adjacent, its position becomes known. It still appears as I, but the I will move wherever the monster goes until it becomes 5 or more spaces away, at which point invisibility once again works fully. This could work differently if the invisible monster is stealthy.
When invisibility is about to time out, give the player a warning message “You thought you saw yourself for a moment” (“You seem to unfade for a moment” if you can see invisible, “You feel introspective for a moment” if hallucinating.
See invisible operates on a scale that determines what level of invisible monsters the player is allowed to see. (A low-level monster that drinks a potion of invisibility should be visible even with weak see invisible, an arch-lich casting disappear should require very strong see invisible.) Invisibility should do this too and have a scale as well (one notable place on the scale being that invisibility is kept only until the invisible thing takes an offensive action). If rings of invisibility and see invisible are made chargeable, this is an obvious avenue for them.
The timeouts for intrinsic invisibility and see invisible from eating a stalker corpse should be very long.
