Destroy the Mutant Leader

There are various ways to complete this quest. First, you can just kill the Master for a quick 10,000 XP in his secret vault under the Cathedral. Second, if you went to Mariposa Military Base first, and you got the key from the locker near the Lou, you could have used that to not confront the Master, but detonate the nuke under the vault. Third, you could have convinced the Master that his plan doesn't work, because all mutants are sterile. He will go crazy, and he will tell you to leave him alone, and find safe ground. He will suicide, by detonating the nuke. The information for the sterility of the mutants can be gained from Vree, the head scribe in Lost Hills.