In the final mission of Halo 3, Sergeant Avery Johnson suffered a direct hit from 343 Guilty Spark’s giant-ass laser beam. Although Johnson would initially survive the attack, long enough to witness the Master Chief destroy the now-rogue 343 Guilty Spark with Johnson’s own Spartan Laser, he would die shortly thereafter.
In Halo 5: Guardians, on the other hand, we see colonies across the UEG going dark along with unsettling deep-space anomalies. No one knows exactly what’s happening, but the sudden appearance of enormous, bird-like Forerunner constructs—formally referred to as Guardians—are no doubt connected. However, the Guardians are more complex than just a colony-destroying menace. The Master Chief, aided by Blue Team, happens to be seeking a Guardian. We saw this in both the original E3 2013 trailer, and in the more recent Halo 5 gameplay trailer at E3 2015.
“Sunaion, the Covenant’s final stronghold. That is where you’ll find the Master Chief, and the Guardian that he seeks.”
Anyway, let’s ask ourselves: why is Chief tracking down an enormous Forerunner construct capable of wrecking total reckoning upon entire worlds? Why track down a construct that unhooded the Chief in the E3 2013 trailer with such impunity?
The answer could rest within that fateful moment at the end of Halo 3. The answer could be that the reason Chief seeks a Guardian, is because the Guardian is controlled by a now-composed essence of Sergeant Johnson. When Johnson died, we saw his body fall into the enormous blue beam shooting from the center of the rapidly collapsing Halo’s control room. That beam, presumably, is composed of Halo radiation—which is what the Halo’s use to nullify the connection between thought and form (and thus wipe out life). That’s not what leads to Johnson’s composition, however. What leads to Johnson’s composition is related to how the Halo Array isn’t the only Forerunner device that uses the Precursor’s technological foundation, neuralphysics, because the Composer also runs off neuralphysics. The Composer and the Halos run off the same technology. Theoretically, one could achieve the same effect with both. That being said, in a situation where a Halo ring is unfinished and unfit for being fired, as was the case with Installation 04b, it may very well be that the enormous blue beam that Johnson’s corpse fell into did end up composing him—disposing his actual body, but harvesting his conscious. So now, a now-composed Johnson is wandering through whatever network he me reside, which is probably the same network that all the Halos synchronize with, and would ultimately upload himself into a Guardian.
Therefore, Chief is hunting a Guardian to find Johnson.