Cortana's speech is the key to her Story Arc.

I have walked the edge of the Abyss.
I have governed the unwilling.
I have witnessed countless empires break before me.
I have seen the most courageous soldiers fall away in fear.
[I was there with the Angel at the tomb]

I have seen your future.
And I have learned.

There will be no more sadness. No more anger. No more envy.

I HAVE WON.

Oh, and your poet Eliot had it all wrong:
THIS is how the world ends.

This message from Cortana has been around since before Halo CE was released. I don’t know exactly when she has ever canonically given this speech, but considering where the story is heading, I feel like these words have more relevance now than ever before.
But what do they really mean?

ya i found it ironic that she is going down the path she is considering that first email from cortana that was part of the Halo: CE ad campaign.

Oh my… I remember pieces of this speech from Chief’s Halo 3 visions. What you’re saying makes a whole lot of since given Halo 5’s plot, but now I want to know more than ever how this speech fits in canonically… Anyone have an idea??

I interpret this literally, as in she has witnessed human (and indeed other species) error and ambition lead time and time again to death, destruction, and futility. To quote Helios from Deus Ex: I should regulate human affairs precisely because I lack all ambition, whereas human beings are prey to it. Their history is a succession of inane squabbles each one coming closer to total destruction.

Cliche, unoriginal, and still has a few flecks of dust on it from having been pulled from a garbage can that has been sealed for nearly 15 years, but at least the motive isn’t entirely black and white.

If Cortana’s story was written 15 years ago I will stand up and slow clap right now. Think about every story that has been told so far in the halo universe…now realize that the biggest story yet has been 15 years in the making. Pretty impressive if you ask me.

Too bad she is setting herself up to fail like the Forerunners.

You know, I still feel like the Librarian had a bigger plan for Humanity as a whole… We’ve known humanity to be the Reclaimers for years, so switching that role over to the AI entirely doesn’t seem right. I have a feeling humanity will still have a role in the Reclamation, somehow… Do you guys thing that the Librarian really intended for Cortana and the Created to inherit the mantle, or is humanity still the rightful reclaimers?

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> You know, I still feel like the Librarian had a bigger plan for Humanity as a whole… We’ve known humanity to be the Reclaimers for years, so switching that role over to the AI entirely doesn’t seem right. I have a feeling humanity will still have a role in the Reclamation, somehow… Do you guys thing that the Librarian really intended for Cortana and the Created to inherit the mantle, or is humanity still the rightful reclaimers?

Nope, she did not and nothing in the previous lore backed that up either. If anything she intended humans and AI to stand as equals, not one lording over the other based on how Chief and Cortana’s relationship developed.

Very interesting…seems like this arc may have indeed been in the making for quite awhile…

Yeah, pieces of that speech came from Cortana in Halo 3, when she was being tortured by the Gravemind. It is hinted in other Media that Cortana may have had access to the Domain at that time, which is why she was able to contact the Chief throughout Halo 3, just as she did on the Argent Moon. It was probably through the Domain that the Gravemind was able to contact the Chief, as well.
If she did have access to the Domain at that time, then she could have been given glimpses of the future; ergo, the speech.
Cortana becoming the “Big Bad” was conceived as early as Halo CE, wherein in an early draft, the Chief returned to the Control Room to find that Cortana was driven mad by power and wanted to control Halo and the Universe, but that was scrapped, or at least delayed, until several games later, after Cortana has suffered through the Logic Plague, Rampancy, and possibly being corrupted by the Domain.
Of course, by this time, many in Halo have fallen in love with Cortana and were furious over her “death” in Halo 4, only to accept it and move on. But now, she has been revealed to be alive and now is the Big Bad, which is no doubt going to cause 343i a great deal of flak.
But not from me. I love the story of Halo 5, and am excited to see where 343i takes the franchise from here. My only gripe with the Campaign was HOW the story was told. It was very poorly done, and the plot twist wasn’t as shocking as it could have been. This was a big let-down for me, as Halo 4 was proof that 343i were master storytellers. One can only hope that Halo 6 is an improvement.
And that they get rid of Brian Reed.