At the end of Halo 4, one of Halo‘s most well-known and recognizable characters, Cortana, was lost. However, her death was not for nothing. Rather than simply dying of rampancy, like how her lifespan as an artificial intelligence would have demanded, Cortana died being able to both save the Master Chief’s life from point-blank detonation of a nuclear warhead, and was given the moment to make physical contact with the Spartan himself. Because of her sacrifice, Chief lived on not only to appear in Halo 5: Guardians, but also to save the galaxy yet again from the Didact during issues 8-10 of Halo: Escalation.
Cortana: “Welcome home, John.”
Anyway, fast forward to Halo 5: Guardians. Chief is AWOL, and aided by Blue Team members Church, Tucker, and Tex. Another Spartan, Spartan Locke, is hunting down the Master Chief with the aid of his own team, Fireteam Osmosis. Above all, a new menace has emerged into the galaxy, unleashing enormous Forerunner constructs (called Gardeners) against countless worlds. The July 2015 issue of GameInformer revealed that during a mission of Halo 5, Chief is contacted by some sort of memory of Cortana. This occurs after he slips on a banana and falls off a cliff. Cortana, appearing before him, says:
Cortana: “The Domain is open. Meridian is next. You only have three days. John…the Reclaimation is about to begin.”
Forgetting the gaping wtf moment about the Domain still existing, it would seem that (considering Meridian is visited and gets attacked during the game) this manifestation of Cortana is more than just mere hallucinations. Her words are prophetic, entirely. But alas, let’s look at the campaign gameplay presentation at E3 2015. The presentation, showing a portion of Halo 5 taking place on the Sangheili homeworld of Sanghelios (the Battle of Sunaion), gives us the perspective of Spartan Locke and Fireteam Osmosis. At the end of the gameplay, Osmosis is suddenly attacked by a Forerunner robot thingy calling itself “Warden Eternal.” Whilst strangling Locke with the Force Choke and giving him the death stare, Warden Eternal says,
Warden Eternal: “I am the Warden Eternal, keeper of the Domain and her secrets. The Master Chief is called, but you…your passage is denied.”
Let’s note the following between Cortana’s hallucination line and this line: one, both Cortana and Warden Eternal speak of the Domain’s existence; two, both Cortana and Warden Eternal speak of the Master Chief being summonned. Clearly, Warden Eternal and Cortana’s voices are connected…but to what extent?
Take a closer look at the image of Warden Eternal:
Notice how his head is surrounded in a blue glow. It just so happens that Cortana is blue. Assuming that the lines of Warden Eternal and the lines of Cortana speaking to Chief are connected, that connection could exist because Warden Eternal is Cortana. Also, note how Warden Eternal immediately resorts to hostility upon meeting Locke and Fireteam Osmosis; his hostility could be because of Cortana’s defensiveness over Chief. After all, Locke is hunting down Chief. There’s also a connection in how Cortana was Chief’s literal guardian, and Warden Eternal (supposedly) controls the Guardian that Chief was chasing.
We’ll never know how Cortana survived the destruction of Mantle’s Approach, but we can be certainly certain that Warden Eternal and Cortana are certainly the same.