Vorpal

General
There's no special animation shown on a successful vorpal. (In other words, it's not literally decapitated.) Aside from a blueish coloration that appears, the monster simply dies.

The following creatures are immune to vorpal's instant death or bane damage, generally because they do not have a head, or do not need a head to survive:
 * Undead (except vampires)
 * Constructs (except Warforged)
 * Beholders
 * Oozes

The following effect work basically like Vorpal on the specific monster type:
 * Disruption vs. Undead
 * Smiting vs. Constructs
 * Banishing vs. Outsiders, Elementals, and Beholders confirm

In addition to having too many hit points, an enemy can resist the vorpal effect if it is otherwise immune to instant kills (e.g., red- or purple-named enemies) or has Deathblock or Deathward. Fortification, however, does not help.
 * Monsters may have Vorpal weapons, and player characters can resist it with Deathblock, Deathward, or lots of hit points.

As of Update 17 the Vorpal weapon prefix now also adds an additional 0.5 damage dice to the attacks of the weapon it is on.

Some items may have a stacking effect which is produced in the event of a vorpal roll, such as Vulkoor's Cunning, and Shocking Blow on Charged Gauntlets.

On random loot, Vorpal can appear on items with a ML requirement of 1. Improved Vorpal appears at ML 15 onward, Greater Vorpal at ML 25 onward, and Sovereign Vorpal on ML 30 items. On current random loot, there are no exceptions to this rule. Older items can have different, usually inferior version of Vorpal (for example, level 18 item with regular Vorpal instead of the "Improved" version).

Trivia

 * The term comes from "," the famous poem by Lewis Carroll, where a hero has a special sword that severs his enemy's head with a "snicker-snack" sound.
 * Not-so-coincidentally, Kasquik the Mighty, a fearsome kobold general in The Shroud, attacks with a vorpal blade that, if he is successful in attacking you, shows gruesome messages in your Combat log:
 * (Combat) You are hit by Kasquick the Mighty's vorpal.
 * (Combat) Snick-snack went the vorpal blade, and it removed your head, killing you instantly.

Named items

 * List of Named Items that vorpal