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.

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 to occur 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 <, and will only take normal damage from the weapon falling back down and hitting you.

Beheading

A monster that has a head is vulnerable to being decapitated by Vorpal Blade; a player polymorphed into a monster that is decapitated will die and be returned to their base form as usual. If Vorpal Blade hits a jabberwock (including a player in the form of one), it will always behead them. 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: so long as it meets the above conditions, Vorpal Blade can behead and kill it.

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". If you would behead a monster with Vorpal Blade and the target has a head but is unsolid, the sword will slice through its neck without beheading them, which is treated as a normal hit.

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 to bisect and instantly kill that monster if they are medium-size or smaller, while a monster that is large or bigger takes double damage. If you use the Tsurugi hit a monster engulfing you and bisection occurs, the engulfing monster will be instantly killed regardless of size.

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.

In addition to granting it to the vorpal jabberwock's clawing attacks, a few other additional monsters may have a "vorpal"-style 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 decaptitate 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 GruntHack's: the vorpal type is chosen based on the object's damage types, which means that a vorpal weapon can behead, bisect or even smash monsters flat. 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 those effects processing unless otherwise stated).

Vorpal weapons that are both slashing and piercing have a 5% chance of decapitating the target, which also damage and potentially destroys that monster's helm, regardless of whether or not that helm would actually be caught in a beheading blow. Vorpal weapons that only deal slashing damage will instead have a 5% chance of bisecting the target, damaging and likely destroying the victim's worn body armor; "vorpal" weapons that deal blunt damage have a 5% chance to smash a monster flat, damaging and possibly destroying that target's shield. All armor destruction is subject to its AC value, meaning that higher AC armor is more likely to survive somewhat intact.

In addition, dNetHack introduces several new artifacts capable of decapitation:

• Release from Care and Crescent Blade can behead their targets; Release from Care has a 10% chanceof 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.
• Levistus's Shadowlock can also behead its target.

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 deferred 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 attack in melee.