Potion of vorpality: consuming it grants you a short duration of a new “vorpal” intrinsic. While you are vorpal, any melee attack you make with a bladed weapon acts like Vorpal Blade in that you may decapitate a monster you hit with it.

Memril, a fantastic insect (so in the x class rather than the a class) which has two bite attacks, each with a chance to decapitate the target. It never generates randomly until the Wizard of Yendor starts harassing the player. It may be a valid target for summon nasties.

Executioner beetle: an insect whose bite has a chance of decapitating its target.

Vorpal bunny, a magenta r monster with a deadly bite: it decapitates you if your HP is at 50% or less, and bisects you otherwise. Probably a reference to the killer rabbit from Monty Python and the Holy Grail.

It should probably only generate in Gehennom, or be unique and generate in a specific area or off-branch of Gehennom, or be psuedo-unique by giving it a birth limit of 1.

Tortles have a Dex-based chance of avoiding getting decapitated by ducking their head back into their shell.

If the hero gets decapitated, they don’t die, but their head falls into their inventory. Thereafter, they are permanently blind unless wielding the head in the offhand slot.

A chaotic artifact axe (or battle-axe, not specified) The Executioner’s Axe: has a small chance of beheading on every successful hit like Vorpal Blade.

Wielding Vorpal Blade in your offhand will never decapitate monsters; instead hits that would have decapitated will instead sever a limb, which can make the monster lose its weapon or permanently lose a claw attack.

Invoking Vorpal Blade gives some sort of message about its keen edge gleaming, indicating that its next hit will have a high critical ratio (perhaps 10% instead of 5%) for increased chance of decapitation.

New object, a metal collar you wear on your neck. Makes you immune to being decapitated by Vorpal Blade. It would probably be worn in the amulet slot.