The Vorpal Buckle kills an extra enemy every time you slay an enemy. So if you kill the 11th enemy, the 12th automatically dies and you will be fighting the 13th enemy immediately after killing the 11th. It can be obtained with Diamonds in the Magic Shop. Once obtained, your Heroine will always have this item.
Few Arai Runewrights were proficient enough to awaken a Vorpal Rune. Its wearer literally slays two enemies for the price of one!
This Vorpal Buckle stacks with the Thegian Pendant of Stealth, cutting the enemies you face each level to just 25 from the original 100. I think it's probably the 2nd Diamond item you should get behind Arai Cloak. There may be arguments for Ring of Damage as the 2nd, but this one is a time saver vs. a push deeper item. Pick which is more important to you when choosing your 2nd item, or just get both =).