Undead warrior shimmer like a wave of heat and are equipped with ghostly blades. As incorporeal beings, they are impervious to all physical attacks, though their own spiritual weapons are just as deadly as any solid implement. Aside from magic, the most practical and adequate method to defeat these blasphemous creatures is to splash it with holy water. This liquid will burst instantly into flames upon contact, the purifying fire engulfing its ghostly form, leaving nothing but a few cinders to scatter in the breeze.

SOURCE(S): Beyond Zork