Many weapons are imbued with the passive "Double Damage to X." The most common is Double Damage to Undead.
These weapons are considered in a class of their own, next to the 3 mobility immunity items (Immobilization, Trip, and Instant Death.) They are suggested to have in your backpack as a Reaper and/or Lich boss killer.
Weapons with Double Damage.
There are currently no Double Damage to X Daggers , Scimitars , or Staffs