Full Circle

Story related. Can only be earned if the mission is completed on Hardened, Veteran, or Realistic. This will be awarded after you finish the eleventh and final mission Life on Hardened or above. If however you've played levels out of order through co-op, you'll get it upon completing the last of eleven levels on that respective playthrough. To check your campaign progress, go to Select Mission and look at the symbol on the right: this indicates the highest difficulty you've completed the mission on.