# Beheading and bisection
Beheading and bisection are properties that appear in NetHack. Both are defined in the code of artifact.h by the same flag, SPFX_BEHEAD, and are properly distinguished from each other in the code of artifact.c.[1]

Contents

1 Description

1.1 Beheading
1.2 Bisection

2 Variants

2.1 SLASH'EM
2.2 GruntHack
2.3 UnNetHack
2.4 dNetHack
2.5 DynaHack and FIQHack
2.6 EvilHack

3 References

Description
Beheading is associated with the one-handed artifact weapon Vorpal Blade, while bisection is tied to the two-handed Tsurugi of Muramasa. Both have a 5% chance of occurring when a monster (including you) is hit with either weapon, and deals HP damage equal to twice their maximum HP; though in practice it immediately kills the monster, this is technically not considered an instadeath due to how damage is calculated.
You cannot behead or bisect yourself by throwing either weapon in the air using &lt;, and will only take normal damage from the weapon falling back down and hitting you.[2]

Beheading
Any monster that has a head, including you, is vulnerable to being decapitated by Vorpal Blade, which is still subject to the amulet of life saving; a player polymorphed into a monster that is decapitated will die and be returned to their base form as usual.[3] If Vorpal Blade hits a jabberwock (including a player in the form of one), it will always behead them.[4] As implied by a comment elsewhere in the code, how many heads the monster actually has (e.g. ettins and their zombie/mummy forms) has no bearing on the beheading effect.[5]
This chance is still accounted for if the target monster has a headless form or is currently engulfing you: the sword instead has a 5% chance of dealing no damage in those cases, as the blow will "miss 

<foo> wildly".[6] If you attack an unsolid monster that has a head with Vorpal Blade, the sword has a 5% chance of slicing through its neck, which is treated as a normal hit.[7]
Despite its name, the deferred monster known as the vorpal jabberwock does not have any beheading attacks, and is simply a tougher jabberwock; many variants that implement the monster often give its claw attacks this property, and usually give it the same weakness to Vorpal Blade's decapitation that other jabberwocks have.

Bisection
When a monster (including you) is hit with The Tsurugi of Muramasa, it has a 5% chance of bisecting and instantly killing that monster if they are medium-size or smaller, or doing double damage to a monster that is large or bigger.[8][9] If you use the Tsurugi hit a monster engulfing you and bisection occurs, the engulfing monster will be instantly killed regardless of size.[10]
Unlike Vorpal Blade, size is the only attribute of a monster that is accounted for when determining if bisection occurs or not - unsolid and corporeal targets alike will be slashed in half if they are small enough, or hit for double damage if big enough.

Variants
Both Vorpal Blade and the Tsurugi of Muramasa retain their beheading and bisection properties in variants of NetHack. In addition, some variants have a vorpal object property that is rarely found on some weapons, and other artifact weapons may feature effects that behead or bisect the target. Some variants grant the vorpal property to the vorpal jabberwock's clawing attacks, and a few other monsters may have an attack that can behead or bisect monsters.

SLASH'EM
In SLASH'EM, the chance of an artifact with the SPFX_BEHEAD flag triggering its effect is raised to 10%—this applies to both Vorpal Blade and the Tsurugi of Muramasa, as well as Thiefbane, an artifact weapon added in SLASH'EM that has the same chance of beheading any @. The potion of invulnerability does not protect you from beheading or bisection, since it only prevents standard HP loss.
SLASH'EM implements the vorpal jabberwock, which is essentially a tougher version of the normal jabberwock and cannot decapitate monsters.

GruntHack
GruntHack has the "vorpal" object property, which can only occur with bladed weapons: Single-handed weapons with the vorpal property function the same as Vorpal Blade, beheading solid non-headless targets with a 5% chance and always beheading jabberwocks; two-handed vorpal weapons will bisect enemies as The Tsurugi of Muramasa does, with a 5% chance of instakilling smaller creatures or of cutting deeply into larger creatures.
GruntHack also implements the vorpal jabberwock and gives this property to its two claw attacks.

UnNetHack
UnNetHack makes use of the vorpal jabberwock and gives its claw attacks the ability to behead targets, though the chance is halved to 2.5%.

dNetHack
dNetHack has the vorpal object property, though it functions differently from most other implementations: the "vorpal blow" is no longer an instakill, but is instead a massively damaging attack with an associated special message chosen based on the object's damage types:

Vorpal weapons that are both slashing and piercing have a chance of decapitating the target.
Vorpal weapons that only deal slashing damage will have a chance of bisecting the target.
Vorpal weapons that deal blunt damage have a chance of smashing a monster flat.
Vorpal weapons that deal piercing damage have a chance of piercing a monster's heart.
In practice, the property also serves as a fallback for artifacts with the same effects hard-coded and/or special-cased into their code, i.e., artifacts have a 5% chance of the effects processing unless otherwise stated. In contrast to vanilla NetHack calculations, a "vorpal blow" from a non-artifact weapon deals 20x + 8d2 in bonus damage, where x is usually the weapon's damage die roll plus its enchantment. If the base damage plus the vorpal blow damage is enough to kill the target, and the vorpal damage is still high enough, a special message is printed as above based on damage type and the target monster is killed. Even if the vorpal blow's damage is not high enough, the target may still die as normal from the combined damage of the blow and the weapon.
Vorpal blows from weapons can potentially be blocked by specific pieces of armor based on their enchantment: a helm protects from beheading (regardless of how much it actually covers the head), while body armor protects from bisection and heart-piercing, and shields protect from smashing. Armor protection in this instance is based on its AC value plus its enchantment: very high-AC armor is more likely to block enough damage for the player to survive, though often at the cost of significant HP loss and the armor being badly damaged, if not destroyed—armor that fails to protect from a vorpal blow will always be destroyed by the hit.
In addition, dNetHack has several artifacts capable of decapitation:

Vorpal Blade has its standard 5% chance of landing a vorpal blow that deals bonus damage equal to (2d12! + x) * 20) + 8d20, where x is the weapon enchantment and 2d12 uses exploding dice.
Release from Care and Crescent Blade can behead their targets; Release from Care has a 10% chance of doing so, while Crescent Blade has the standard 5% chance.
The Lifehunt Scythe will behead targets that are hit while sneak-attacking with it, with the normal 5% chance per hit.
Snickersnee and the Kusanagi no Tsurugi are capable of beheading monsters; the Kusanagi has a 10% chance, while Snickersnee has the standard 5% chance.
Levistus's Shadowlock can pierce its target's heart.
On the other hand, monsters with vorpal attacks can still instakill with them unless the target is wearing a helm or has a preservation engine. Those monsters are listed as follows:

Dracae Eladrin
ancient of death
shayateen
twitching four-armed changed
uvuudaum, via its brainspike attack
DynaHack and FIQHack
In DynaHack, the vorpal property functions the same as in GruntHack, except that it no longer occurs on randomly generated objects. The same is true in FIQHack.

EvilHack
EvilHack also uses the vorpal jabberwock, which has decapitating claw attacks and behaves very similarly to GruntHack's version of the monster.
EvilHack's shambling horror can roll beheading or bisecting attacks among its randomized traits, making them potentially lethal to approach in melee.

References

↑ src/artifact.c in NetHack 3.6.7, line 1250: A comment a few lines down reads "not really beheading, but so close, why add another SPFX"

↑ NetHack commit 6ce2277 - "Fire Brand vs sliming"

↑ src/artifact.c in NetHack 3.6.7, line 1337

↑ src/artifact.c in NetHack 3.6.7, line 1296

↑ include/artilist.h in NetHack 3.6.7, line 145

↑ src/artifact.c in NetHack 3.6.7, line 1304

↑ src/artifact.c in NetHack 3.6.7, line 1312

↑ src/artifact.c in NetHack 3.6.7, line 1264

↑ src/artifact.c in NetHack 3.6.7, line 1274

↑ src/artifact.c in NetHack 3.6.7, line 1254: Code for bisecting engulfers from within; all engulfing monsters in NetHack are at least large in size or bigger

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