Vorpal

There's no special animation shown on a successful vorpal. Aside from a blueish coloration that appears, the monster simply dies.

If the enemy has more than 1000 HP, or is otherwise immune to instant kills (e.g., red- or purple-named enemies) they instead take 100 points of bane damage.

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.

These 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

Also, the Deathblock and Deathward effects prevent vorpal. Fortification, however, does not.

Trivia

 * The term comes from "Jabberwocky," 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.