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.

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 Louis Carroll, where a hero has a special sword that severs his enemy's head with a "snicker-snack" sound.
 * Not-so-incidentally, Kasquick 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.