Just have a look here and go to 21:10
Hey I’m guessing you completed the Halo 4 and you are shocked about what happened to Cortana , but there’s still a lot of hope !
https://forums.halowaypoint.com/yaf_postst137119_Cortana-s-Role-in-Halo-4--In-Depth---Spoilers.aspx
Check that thread out ! You can thank me later ! 
Sweet! Thanks brother!