Piercing Damage is a physical form of damage which likely bypasses Defence.
Longswords, Greatswords, Daggers, Faeblades, and Longbows are natively capable of delivering Piercing damage. Staffs, Hammers, Sceptres, and Chakrams do not natively deliver piercing damage, though they can be custom-crafted to do so; the former two via Piercing Shafts and/or Weapon Gems, and the latter two via Piercing Handles.
The gems that add Piercing Damage to custom-crafted weapons are:
The Fateless One can use potions to add Piercing Damage to his/her attacks, and to augment existing Piercing Damage delivered thereby. These potions are: