Cancelled, non-unique warpers (and possibly some unique warpers, like non-magical quest nemeses) shuts down their warping. This means that cages in Gehennom can contain liches that are cancelled and thus incapable of warping out.
A death ray zapped over a monster egg on the floor kills it, making it so that it will never hatch.
Possibly, this effect could also be achieved by erodeproofing or canceling the egg.
Throwing holy water at demons should be buffed way over its current 2d6 damage, because it’s a fun interaction. Instead, it should severely weaken them, cancel them, stun them, and/or do a large amount of damage (perhaps 50% or some proportion of their max HP, possibly exempted for demon lords whose fights it would trivialize).
In variants where the spell of cure sickness can be cast directionally, casting it at Pestilence has some detrimental effect on him, probably hitting his HP and possibly stunning or even cancelling him.
Antimagic lamps, which cast an magic-denying field in their light radius. Your magic doesn’t work, but neither does anyone else’s, with everything behaving as if cancelled.
Anti-magic fields in Gehennom can attempt to cancel you or your gear, and apply the cancelled status effect.
Getting cancelled should end a bunch of magical effects:
Canceling magical armor not only sets it to +0 but turns it into a nonmagical counterpart, which consists of new items such as “faded cloak”, “faded boots” etc. This would probably also rename “faded pall” to avoid confusion.
When a wand of cancellation is placed into a bag of holding, rather than blowing it up, it instead uses up (all of?) the wand’s charges and turns the bag into a sack.
For consistency, bags of holding should probably turn into sacks when zapped with cancellation as well.
This forces potentially interesting choices on cursed bags of holding in bones files, since now cancellation cannot safely be used to loot all the contents and get the bag too.
There should be a spell (probably on the player’s side, possibly on the monster’s side) that dispels magical buffs such as protection and reflection. Or else, if adding a new spell is unwanted, this effect could be folded into cancellation instead.
Cancelled monsters have lower monster magic resistance.
Some way to do a giant mass-cancellation which cancels every monster on the level without a chance of resistance or a saving throw, but also uncontrollably cancels every item on the level, including the player’s inventory and items in containers.
Hitting a statue trap with cancellation removes the trap, so that it is just a regular statue.
A new R-class monster (possibly “nullifier”) whose attack cancels monsters it hits. Possibly it does some sort of object cancellation as well.
Creatures that get caught in a wand’s explosion (from the wand actually breaking, not from the regular effect of the wand) become uncancelled, unless the exploding wand is a wand of cancellation. Notably, this can be used by the player to remove the Cancelled status.
Canceling a wand of cancellation turns it into a random wand.
Firefly, an a monster that is a light source, but a variable one. It ranges from being totally unlit to emitting light in a radius of 3 for a brief time. Is poisonous to eat, and with very low nutrition; has a mild bite attack. Tends to spawn in dark areas.
Internally, the deciding factor when a firefly should light up should probably use its mspec_used variable, which enables them to light up again when it hits 0. Also, cancelling a firefly should prevent it from lighting.
Monsters can now zap cancellation at the player; effects of cancellation on the player include draining Pw (and possibly maximum Pw if Pw hits 0) and removing magical temporary intrinsics such as invisibility, speed, and detect monsters. It does NOT cancel the player’s entire inventory, as this does not happen when the player zaps cancellation at a monster.
Cancelled monsters can quaff potions of restore ability to uncancel themselves.
Shopkeepers can be cancelled. When cancelled, they won’t buy or sell any items. They will still react to theft, but they can’t be paid and they don’t offer any money or claim ownership of items you drop in their shop.
Bumblebee, very like a killer bee, but its poison sting is worse. However, when it stings you, it becomes cancelled and can’t sting you again; a cancelled bumblebee additionally loses 1 HP per turn.
Fairies, a n monster similar to pixies: very small, flying, capable of hiding under objects. However, they are lawfully aligned and (?) always generated peaceful. If you chat to a non-hostile fairy, it will heal you a moderate amount (as in the Legend of Zelda) and become cancelled (a cancelled fairy can no longer heal you).
Artifact unaligned touchstone called Grinder: rubbing it on or applying it to an object repairs one level of rust or corrosion but decreases enchantment by 1 (which can go negative). Invoking it cancels a single object.
Cancelled trolls won’t rise from the dead.
Zapping a cancellation beam downwards while standing on a magical (magic, teleportation, polymorph, etc) trap will remove the trap.
Cancelling a unicorn horn makes it non-magical and removes its curative properties.
Cancelling shades should remove their incorporeal ability to not be hit by anything, and they should then be hittable normally.
Mind flayers cannot tentacle you if they are cancelled.
