There are spoilers for Halo Infinite and some of the books. I highly suggest playing the game before reading this.
These are my thoughts on the two topics that I’ve seen the most subject lines discussing. Cortana, and the Harbinger.
I think Cortana’s actions could have been better explained, but I don’t think it was out of left-field. In Halo 4, we saw one fragment of her stay with Master Chief, and then fade out of existence, and we saw in Dominion Splinter that the rest of her fragments coalesced in the Domain and formed a new Cortana, minus the personality fragment that saved Master chief from the nuke at the end of Halo 4. I believe the fragment of Cortana that saved Master Chief before being lost contained Cortana’s capacity for empathy & sympathy. That’s why the remainder became tyrannical in Halo 5. That version of her had all of the intellect and pride of the original, but not the core of humanity to curb her ego, and she pursued the means that she believed was most likely to result in her desired outcome, not concerned with the collateral damage done, the innocent lives lost in her self-imposed quest.
In Halo Infinite, we learn that Cortana destroyed the homeworld of the Jiralhanae (Brutes) as a punishment for Atriox’s refusal to follow her, which is what sent him into a genocidal rage, intent on exterminating humanity, because Humanity created Cortana. This version of Cortana still lacked the ability to have empathy, but she did recognize that her plan failed, and after a brief test of Atriox’s mindset (when she posed the question of if he would live differently if he could do it again), she decided to take him out with her, blowing apart a large part of Zeta Halo to stop him. That’s not so much a self-sacrifice as vengeance. It’s sort of a “if I’m going down, I’m taking you with me.”
One thing that did strike me as odd is how there were echos of Cortana after saving Master Chief at the end of Halo 4, something a fragment of her did that the others constituting the Cortana seen in Halo 5 & Infinite wasn’t present for. It’s possible that the speech was one she’d prepared before splitting off her rampant fragments. The only hint of this possibility is Cortana’s final echo responding to the Weapon’s statement that she’s just an echo, having predicted what a copy of herself would have said, and the timing of it. Notice that the extent of her apology is that she tried to do it alone instead of working with Master Chief. Cortana never apologized for the genocides she committed. Even in the end, she still doesn’t have that piece of humanity that she lost at the end of Halo 4. Her final sentiment was bragging about how good her choice was when she chose Master Chief.
They seemed to imply that the Weapon might take the name Cortana, but I don’t think that would be a good direction to go, because her relation to Cortana is more like an identical twin, rather than being the same person, and it doesn’t seem right to take a sibling’s identity for your own.
The Harbinger is interesting. When she first meets Master Chief, the first thing she tells him is “we are not enemies.” She then identifies herself as the Harbinger of the Truth, denouncing Forerunner lies, and then says the Reclaimers are not the future, and that the Endless will ascend.
This seems to be a reference to the Mantle of Responsibility. The Precursors’ Mantle of Responsibility is the duty of one race to safeguard the opportunity of other races to reach their maximum potential. It’s not a militant rule, or even governance, merely a laissez fairre protection from being exterminated or perpetually held back.
The Harbinger augmented Forerunner weaponry, improving it to burrow into the Halo. That is a significant feat.
Cortana and the monitor, Despondent Pyre, both considered the Harbinger and the Endless as “worse than the Flood”. That is significant as well. I wander if the terrible threat that couldn’t be destroyed, and could merely be contained was the physical forms of the Precursors, Locked away, never to be spoken of or accessed again. Everything except the fear of them being lost to time.
There were multiple instances through the game that hinted at a connection to the Flood. I’ll be playing through the Campaign a few more times to iron out my theories, looking for clues I may have previously missed.
A big thing I noticed is that in the final fight against the Harbinger, the Weapon realizes that something is helping her, but she doesn’t know what. In that same fight the Harbinger seems to be over-the-top theatrical with her taunts, as if it were a stage production. I think the Harbinger might be part of the Precursors’ test of worth for the Mantle.
On a side note, when we see the Harbinger without her helmet, she does bear a passing resemblance to the Primordial, though the latter was already a Gravemind with multiple bodies in its form explaining the differences. I think it’s possible that when the Forerunners rebelled, they put many of the Precursors’ physical avatars into stasis, and that the Harbinger could be one of them. I wouldn’t bet money on that, but I think it’s a possibility.
The visuals of the game are glorious. I really liked how the Sangeheilli (Elites) regained their elite posture and movement, I’m not typically one to complain about artistic choices, but I really hated the mandibles that looked like they were biting their own eyes, and their brutish lumbering gate in Halo 4.