The Librarian was unlocking Chief's geas. In the short term, it helped the Master Chief defeat the Didact by making him immune to the composer's effects. Though, we still don't know the true purpose of the geas she unlocked. [i]"Reclaimer, you are the culmination of a thousand lifetimes of planning." "Planning for what?" "He has found us." -Cutscene, Reclaimer, Halo 4[/i] Both the Librarian and Didact are Forerunners. However, the Librarian we see in Halo 4 is just a digital imprint of her personality, as the real Librarian perished on Earth when Halo was fired.