The full story
America Chavez is running for her life. She can open star-shaped portals between universes—a power she can't control—and something is chasing her through reality after reality. In Universe 838, the Sorcerer Supreme (a ponytailed Strange variant) tried to take her power to stop Thanos. He failed. The monster caught up. America accidentally portals herself and that Strange's corpse into our universe, Earth-616.
Strange saves her from a one-eyed octopus demon in the streets of New York. He recognizes the runes on the creature—witchcraft. He goes to the one witch he knows: Wanda Maximoff.
Wanda sent the demon.
The Darkhold has corrupted her completely. The Book of the Damned showed her what she lost—her children, Tommy and Billy, created during WandaVision—exist in other universes. She wants America's power to travel the multiverse and find a reality where she can be with them. She's willing to kill a teenager to do it. She's willing to kill anyone.
The Scarlet Witch attacks Kamar-Taj. The battle is brutal—sorcerers dying by the dozens, Wanda walking through defensive spells like they're cobwebs. Strange and America escape to Universe 838, seeking help from that reality's Illuminati: a council including Mordo, Captain Carter, Black Bolt, Captain Marvel (Maria Rambeau), Reed Richards, and Professor Xavier.
Wanda finds them. She "dreamwalks"—possessing her variant in 838—and slaughters the Illuminati in minutes. Black Bolt's mouth vanishes; his scream liquefies his own brain. Reed Richards shreds like string cheese. Captain Carter is bisected by her own shield. These are some of the most brutal deaths in MCU history, and Wanda barely breaks a sweat.
The horror escalates. Strange dreamwalks into a dead variant's corpse, animating his own rotting body with souls of the damned. America, pushed to her limit, finally controls her power. She doesn't fight Wanda—she shows her. She opens a portal to a universe where Wanda has her children, and those children see their "mother" for what she's become: a monster covered in blood, terrifying.
Wanda breaks. She sees what the Darkhold made her. She uses her power to destroy every copy of the Darkhold across every universe, bringing Mount Wundagore down on herself. The Scarlet Witch is gone.
Or is she? A flash of red in the rubble. Wanda's fate remains ambiguous.
America stays at Kamar-Taj to train as a sorcerer. Strange, having used the Darkhold himself, discovers its corruption: a third eye opens on his forehead. He's been changed. The mid-credits scene shows Clea, a sorceress from the Dark Dimension, recruiting Strange to fix an "incursion" he caused—universes colliding because of his actions. In the post-credits scene, the street vendor Strange annoyed earlier ("Pizza Poppa") confronts him again over a stolen pizza ball; Strange curses him into repeatedly slapping himself in the face.