The Black Shard Arm replaces Billie Lurk's right forearm with a construct of the Void. It is a unique gift from the Outsider, given to her after she rescues Daud from the Eyeless Gang. The artifact draws and replenishes its power from the Void itself, rather than having her rely upon Piero's Spiritual Remedy or Addermire Solution.
The Black Shard Arm allows Billie to use Displace and Semblance, and provides the energy for Foresight. It is also able to interacts with hollows, eliciting a comment from the Outsider. When Billie collects the Twin-bladed Knife, the two objects merge.