This is given by Kells after speaking to Dr. Li during "Liberty Reprimed". He'll tell you that the supermutant Virgil must be killed. You can spare Virgil then lie to Kells that you killed him. Otherwise, if you delivered the Experimental Serum to Virgil and he's turned back to a human, you can attempt to persuade Kells that Virgil has been cured and there's no longer any reason to kill him. Kells will then give you the Visionary's T-60c Helmet as reward.