Formless Void Essence
In patch 8.3 Formless Void Essence lets you copy Essences from other players
It's a tradition in World of Warcraft going all the way back to Cataclysm and the Dragon Soul raid that the last raid -- especially if it has any sort of Old God theme like Dragon Soul or Siege of Orgrimmar did -- will have weird items with unusual procs and powers that fit the theme of chaos, madness, and corruption of those selfsame Old Gods.