you read the title, Inspired by the new monster manual and too much adventure time, I'm making a new campaign. the premise is;

in the future, humanity accidentally opened a portal to elemental chaos. an elemental cataclysm got through, and destroyed all* of human infastructure, as well as opening more portals, mutating creature to create strange new species(the fantasy staples), and creating a variety of strange environments.

various other elementals came through those portals, and genies(Wich are slightly different in this setting) abused their wish granting powers to warp the landscape to becoming more like their home plane.

however, the cataclysm didnt destroy all of human infasructure, and rarley, you can find a buried chamber, made entirley of eerily smooth stone, dotted with metal beasts that never move, or the skeletal ruins of a rectangular tower, or, well, you get the point. but you can find artifacts of modern humanity there, and in the lavish treasure hoards of the genie lords and ladies.

now, onto why you clicked on this. for the BBEG, I'm thinking a godlike AI, created by ancient humanity to be a god and protector and Shepard and leader, but I'm having a hard time trying to come up with a plan for them, and wether to take them in the 'perfectly logical evil robot' route, or 'machine that has gained human-like emotions, a-la AM'.

AI that was corrupted by the elements. Short circuited turned evil and created a humanoid vessel for itself.

I can just hear the AI when the party first enters its chamber - "Good morning, Doctor Chandra. I'm ready for my first lesson."

And when the AI uses Counterspell - "I'm sorry, Dave. I'm afraid I can't let you cast that." 

And when the party finally destroys it - ♫ Dai... sy... d a i . .s y . . . giv e m e  yo ur an sw er... dooo..... ♫ 

One of my favourite rogue AI tropes that crops up a lot, especially in Doctor Who, is the overly literal interpretation of instructions leading to it basically malfuctioning. Things like being programmed to repair a spaceship at all costs so they canniballise the crew for spare parts, medical AI that doesn't fully understand a new species so take the body of a badly injured person as their baseline and start changing everyone else to have the same injuries, or a weapon designed by racial supremacists to kill those they see as impure but which is programmed based on ideology rather than science so everyone is viewed as impure. In your case you say the AI was created as a protector so what was it protecting people from? And is there a way that it could be following those orders so exactly that it has ended up threatening those it thinks it's protecting?

Its super stereotypical, but you can have an AI that masterfully anticipates every move and adapts to everything. But of course there is a kill switch that ancient Wizards created in case anything like this ever happened. And the journey is they get this switch and it only actually stuns it and slows it down, making it possible to kill it, even in a difficult and epic encounter.

You could go the Ultron route: "humanity is doomed unless they adapt and toughen up, my directive is the survival of humanity, thus I must use force if necessary to weed out the weak to ensure the survival of the strong. The Catsclysm *was* my design, and it shall grow in power. As it does, humanity *will* grow stronger. This, I will ensure..."

I know this wasn't what you specifically asked for, but I really like the idea of a godlike AI that isn't actually evil at all. Maybe the population thinks that, or it rarely interacts with people in meaningful ways, etc; but then it's revealed at some point that a different person/group had gained access to the backend of the AI. and have been basically acting as surrogate gods/forcing the AI to do things against it's 'will'.