Strange set up Westview as a paradise/prison for Wanda b/c he knew she could go supernova after Vision's death. (like how he told Loki he watches everyone he deems a threat to Earth)
He had "Agnes" go there to observe everything, but had no clue how powerful and evil she actually was.
He comes in at the end to help Wanda and Vision fight Agnes, and probably one of her demon dudes that everyone has theorized. no idea about this, its just been the popular guess from day 1. Probably evil Pietro too.
Agnes tells Wanda that Strange was behind it all, and she turns on him. I feel like her kids probably get in the fight too. Maybe the fast one fights Pietro and the other one somehow calms Wanda down b/c they're hinting he's like her. maybe he kills Agnes... the fukk knows... Monica is in the fight as well.
Strange tells Wanda he was just looking out for her and that they have to shut the hex down before it engulfs the planet. It's some Professor X/Jean Grey type shyt where you know Wanda could snap at any second. Vision knows he cannot exist outside of the Hex so he either sacrifices himself, or he makes Strange kill him. This sets Wanda off, and she does some kind of blast that creates mutants and tears open the multiverse.
Show ends with Strange basically saying something along the lines of Thor's "What did you do" in IW.
Wanda (and the demon dude) get away somehow, and both come back in Strange's movie (Olsen is confirmed in the movie) She's the antagonist of Multiverse of Madness, until the third act where she aligns back with Strange and they close the multiverse, but mutants are now in the MCU. Devil dude is the other villain b/c it's a horror movie directed by Sam Raimi, there has to be that type of character.