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

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. 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.

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 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.

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, with the "vorpal blow" type 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 to smash a monster flat.
• Vorpal weapons that deal piercing damage have a chance to pierce 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 instakill 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 the 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 artifact weapon.

Vorpal blows 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 protectiveness is based on its AC value plus its enchantment, meaning that very high-AC armor is more likely to block enough damage for the player to survive—this is still likely to come at the cost of badly damaged (if not destroyed) armor and a significant or even massive loss in HP, and armor that fails to protect from a vorpal blow will always be destroyed by the hit.

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 pierce its target's heart.

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.