Two (or more, in this case usually elemental) artifacts, each of which is a counterpart of the other. Typically they are either identical but distinguished by their users and names, or Yin and Yang-like different.
One common plot element is the Hero having to acquire one of these to allow him to defend against and/or defeat the Villain, who has the counterpart. It's also common to distribute them [...] [tvtropes.org]