Vorpal is a sword enchantment, added by RedPower 2, that increases the chances of mobs and players dropping a head on death. You can have up to 4 levels of this enchant. This does not seem to affect Wither Skeletons (you can use Looting for that), although it adds a chance of doing severe damage to mobs, the greater the enchant, the greater and more likely the damage. Any additional functions are purely for decorative purposes (or for Creeper face fireworks). It does, however, encourage Zombie Pigmen to drop normal Zombie heads.
The name Vorpal probably refers to the Lewis Carroll nonsense poem, the Jabberwocky, in which a beast (the Jabberwock) is beheaded with a Vorpal Sword.
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