Please keep in mind that this is just a theory.
Let me go into detail.
ONI was constantly trying to find new weapons and technology. The more info they had, the greater advantage they had against the Colonial resistance and eventually the Covenant. They recruited Dr. Catherine Halsey, a young prodigy, to create the Spartan and Mjolnir programs in hopes of cutting down the leaders of the resistance. As time went by, they stumbled upon Forerunner technology. With Halsey studying the Spartans and overseeing the Mjolnir armor, she was sent to research this new technology in hopes of . She created Cortana, under orders of ONI, to decode and understand any Forerunner items she could get a hold of. ONI then sent her to research the new technology on Reach, which seemed the safest planet at the beginning of the Covenant war. She was then forced to finish her research before she could leave Sword Base. While she finished her research, with Noble Team fighting holding the Covenant at bay, she uploaded everything she learned into Cortana. ONI told Halsey to get her research into the hands of a Spartan capable of protecting it. She left the decision up to Cortana, who chose John-117.
After fleeing Reach, Cortana was fortunate enough to stumble upon Installation 04. As she was connecting with the ring, she also was gathering knowledge for ONI. When she was connected with the control room, her search for knowledge led her to the discovery of the containment of the Flood. She warned John of Captain Keyes’ mission, and while he was gone, she searched even more. The more she learned about Halo and the Forerunners, the more she started to set her own plans into motion.
After returning to Earth, Cortana learned of ONI’s plan to use the Forerunner technology to police the universe from any enemies. So, she only told them enough to satisfy them. She told them where to find more Forerunner technology, which she would eventually end up activating in Halo 5. She opted to continue with Chief in order to gain more knowledge of the Forerunners from Installation 05 and to stay out of reach of ONI. When she discovered the Gravemind, she had many questions for it about the Forerunners. Instead of continuing with Chief to stop Truth, she stayed on High Charity in order to chat with the Gravemind. Cortana wasn’t prepared for the Gravemind’s logic plague and was drastically trying to find a way out. She sent Chief a message warning him of Gravemind, and to lure him back to her. Chief took the bait and rescued her from the clutches of Gravemind. She was saved from one end, but sent to another as the logic plague slowly worked in her.
While Chief was asleep in the Forward Unto Dawn, Cortana was trying to find a solution to the logic plague. It was pure luck that she was able to find the Forerunner planet Requiem. She gained new hope for finding a cure on this “gold mine” of a planet. Every terminal, every interface, she searched for something new, something that could save her. She eventually stumbled upon the Domain and a way to enter it. But, she needed a distraction. She knew that John would overcome any obstacle in order to save mankind. So, she released the Didact. While helping John, and also feeding him fuel to save her as well, she set her plans in motion. When she saw the right moment, she split herself to help John and to open the Domain. With her new found power from the Domain, Cortana was able to help John stop the Didact and save mankind. She was also able to have a heartfelt goodbye with her most cherished friend before developing her powers in order to continue with her plans.
Cortana was then able to control the Prometheans and have them turn on the Covenant. She was determined to beat ONI to their goal. With all the distractions for ONI, she sent out a message to John and led him and Blue Team to her. She wanted to police the universe with John and Blue Team at her side, and to show ONI that she was in control now.
I’d like some feedback on this.