I'm assuming "Doppleganger" because whoever you cast it on gets the enmity you generate instantly.

So basically, if I pull the boss after hitting the tank with "Shadewalker", my clone, or Doppleganger, is seen by the enemy as a mirror on the tank. Therefore, the tank gets all my enmity. This is helpful to allow the tank to go DPS stance and deal more damage before needing to apply Enmity combos at the start, so long as "Shadewalker" is active on him.