If you haven’t played or finished Halo 5 yet, don’t read this, because you’ll be spoiled!
AI are a big deal in the Halo universe, and Cortana is easily the most important one in the whole series. Since she first showed up in Combat Evolved, players have formed a bond with her, both as themselves and as the Master Chief. The two have seen so much together that Chief is willing to go through an alien portal to who knows where. Halo 4 added an underlying tragedy to their friendship by having Cortana’s instability be the thing that ultimately drove them apart, though more by necessity of the dire situation than by choice. To save the Chief, Cortana had to give in to her Rampancy and use what little power she had to save him and create a body to actually touch him for the first time.
Halo 5 brings Cortana back, but not as the quippy AI construct that she’s usually been. Being transported to the Forerunner world Genesis has essentially cured her and unlocked her full potential, which you think would be awesome, right? Yeah, no, she’s kinda off her rocker and decides to use giant Guardian constructs as warnings to any race in the galaxy that doesn’t follow her rules.
Even worse, she reaches out to all the AIs in the galaxy, the majority of which pledge their allegiance to her. With all that power and knowledge in her hands, plus the combined ability to be anywhere in the blink of an eye, the galaxy could very much be screwed by stealth tech attacks or malfunctioning equipment. And that’s to say nothing of what could happen to the AIs who didn’t immediately accept her offer, like Roland on the Infinity. And on that note…