Okay, here's the idea, a weapon that can't ever possibly kill an undead enemy!

Special feature: this weapon is made of solidified darkness that has taken on a crystaline form, it deals negative energy damage rather than slashing damage.

effect 1-Negative energy damage on each hit

effect 2-Massive negative energy burst on criticals

effect 3-If this weapon heals the enemy targeted, you will heal for an equal amount to what the enemy is healing, additionally, spellsword, filigree, and runearm damage is automatically converted to negative energy damage while this sword is equipped.

effect 4-Legendary Soul Eating: On a Vorpal strike, the dark energy that inhabits this weapon will tear at the soul of your foe, inflicting 2d6 negative levels to your target. The Soul Eating weapon will feed this stolen life back to you in the form of 1000 temporary hit points that last for one minute or until depleted by incoming damage. (This temporary hitpoint effect does not stack with Bloodfeast or Legendary Affirmation).