I know that some people believe that John 117 has a geas that led him to the Librarian during the events of Halo 4 but I believe this is absolutely incorrect. At no point during the game does John ever independently take an action that is guided by anything other than duty and a desire to stop the Didact. It is always some external force or entity that pushes John toward the Librarian. In the level in which he meets the Librarian he is only focused in taking down the cannons, and has no other focus. The only reason he even approaches the area that the Librarian speak to him is to retrieve his equipment. When he does meet the Librarian he doesnât know who she is and she doesnât reference him as more than a point where events collide. It is more likely and logical that it is I fact only Doctor Halsey who had a geas. She recognizes the Librarian on sight, takes active steps to find the Librarian, and her geas can also explain John ending up where he was.
Halsey created the Spartans, meaning she screened them for traits and most importantly their genetics. Part of the reason behind having John meet the Librarian was not only to inform him of the Composer but to activate a dormant part of his genetics. The Librarian in no way says that only John has that genetic mutation only that he was the one brought to her with the AI. It is logical then to conclude that the genetic mutation that could make a person immune was part of the genetics Halsey required of the Spartans. It is also likely one of the genetic qualifiers that was thrown out for the later generations of Spartans so the candidates werenât so narrow as Halsey had nothing to do with the later generations and would to insist on it.
It is also because of Halsey that John would have been in that exact position. Itâs more likely that John wasnât guaranteed to be the Spartan that would be standing before the Librarian, that all Spartan IIs were candidates to end up in that position, but it was John because of Halsey. Halsey has an obsession with John which likely put him on the path of events that led him to the Librarian. She not only had decided he was going to be Squad Leader, placing him in charge of all Spartans, but her obsession with him transferred to Cortana.
We all know that Cortana was made through illegal acts which could be attributed again to the geas. Halsey may not have only used her own mind because sheâs egotistical but because she knew she needed to ensure that the AI had the same geas guiding her. Remember that the Librarian said that he had a specific AI, indicating that the AI was somewhat predetermined. The Librarian could not have predicted exactly Cortana but she could influence how it was made to ensure simple things about it like it being made from Halseyâs brain. Cortana was likely also supposed to aid in the selection of the Spartan that would meet the Librarian, using the information gathered over the years to choose the correct soldier but, because of the inherited obsession, chose John.
It should be remembered that while John is a good Spartan and Lucky he is not the best Spartan. In fact heâs average. There are stronger, faster, and specialized Spartans. Fred is better at hand-to-hand, Sam was stronger, Linda is a better shot, Grace was better with explosives, Kurt was better at sensing trouble or ambushes, and Kelly is faster. Even John thought Fred was a better Spartan overall but it is Halseyâs obsession thatâs evident in the journal included with Halo Reach that she had no interest in any of the other Spartans and had already made up her mind.