Remove up to 3 damage counters from one of your Assassins or Sith Apprentices and put 3 corruption counters on that Assassin or Sith Apprentice has 6 or less corruption counters on it and only during your build step.
Keywords
Shields
Shields X: A cumulative, static ability that means, "The attacking unit gets –X power against this unit." Shields only matters when attack dice are rolled. If you give a unit Shields after your opponent has rolled for damage, it doesn't affect the roll.