Vorpal Strike

A vorpal strike refers to rolling a natural 20 as your weapon attack roll, and confirming the critical hit.

It's referred to as this as this rule defines how Vorpal weaponry works, but it applies to many other special effects in DDO, such as:
 * Banishing
 * Cursespewing
 * Disruption
 * Manslayer
 * Smiting

Few effects exist that can extend your vorpal range to both 19 and 20:
 * Perfect Single Weapon Fighting - applies while Single Weapon Fighting
 * Sniper - applies while using Great Crossbow
 * Bloodbath - applies while Two Handed Fighting