Magic cancellation

Magic cancellation, or, shortly, MC, is a property of some pieces of armor which protects against a wide variety of special melee attacks by monsters, such as poisonous killer bee stings and lycanthropy caused by werejackal bites. A precise list of attacks that magic cancellation protects against is provided in the section below. Generally speaking, magic cancellation protects against special non-damage effects (such as sliming) caused by a monster attacking and touching the player with some part of its body.

A player is protected by an aura of magic cancellation if one of the pieces of armor worn has magic cancellation. Each piece of armor has a magic cancellation number (MC number) ranging from 0 to 3, where 0 is the lowest (worst) and 3 is the highest (best). Note that magic cancellation is not cumulative — the player's magic cancellation is the maximum of that provided by any single piece of armour worn. This means that if you are wearing a chain mail with MC 1, and a dwarvish cloak with MC 2, then your magic cancellation is 2. Having the highest (best) magic cancellation means that only 1/50 of special, non-damage body-to-body melee attacks will succeed (assuming they would hit otherwise). This makes magic cancellation a high priority trait.

Magic cancellation must not be confused with cancellation caused by a wand or spell, nor with magic resistance. See section Difference between magic cancellation, cancelling a monster, and magic resistance for furter details.

How it works
Understanding how magic cancellation works requires understanding of how a attack works in Nethack. An attack has two characteristics: attack type (e.g. bites and stings) and damage type (e.g. physical or poisonous). A soldier ant for instance, has two attacks: a bite which does 2d4 physical damage (i.e. ordinary damage), and a sting which does 3d4 poison damage. Magic cancellation will not protect against the 2d4 physical damage from the bite, nor will it prevent the 3d4 damage caused by the sting. It will however protect against the special effects from the poisonous sting, which randomly includes hitpoint reduction, attribute reduction, and instadeath.

There is no obvious pattern to what magic cancellation does and doesn't protects against. Shortly put, magic cancellation protects against certain attack types that deal a certain type of damage. This relation is shown in the table below. To be prevented by magic cancellation, both the attack type and damage type must be in the orange area of the table. If either the attack type or damage type is outside the orange area, magic cancellation will have no effect on the attack.

List of magic cancellation numbers by armor
The relationship between magic cancellation number and chance of a body-to-body special attacks success is as follows:

All cloaks, some pieces of body armor, and the cornuthaum provide non-zero magic cancellation. All other items in the game provide zero magic cancellation. The following list contains all the pieces of armor with non-zero magic cancellation:

Most players strive to have maximum magic cancellation, but some favour the benefits conferred by a cloak of displacement (MC 2).

Difference between magic cancellation, cancelling a monster, and magic resistance
Magic cancellation is a property of armor that reduces the probability of certain special monster melee attacks of succeeding. A monster can also be cancelled by zapping a wand or a spell of cancellation. This is somewhat different from magic cancellation. Cancelling a monster completely removes the monster's ability to perform its special melee attacks, while magic cancellation only reduces the probability of those attacks succeeding against the player. Also, the attacks affected by cancelling a monster are not exactly the same as those prevented though magic cancellation. See the article on cancellation to compare effects.

Magic cancellation is not the same as magic resistance. Magic resistance is an extrinsic that protects against spell, wand, and trap effects and damage.

Common misconceptions

 * Magic cancellation does not protect against paralyzing passive attacks nor gaze attacks, which include the attacks of floating eyes and Medusa.
 * Magic cancellation partially protects against disenchanters; it prevents the disenchantment though the claws, but not through the disenchanters passive attack.
 * Magic cancellation does not protect against the hissing attack of the cockatrice.