Wanda Maximoff is revealed as the architect behind the monsters pursuing America Chavez. Corrupted by the Darkhold, she intends to kill America and siphon her power to permanently reside in Earth-838 with her sons, Billy and Tommy.
When Strange refuses to surrender America, Wanda attacks Kamar-Taj, killing dozens of sorcerers. Fleeing through a portal, Strange and America land on Earth-838, where they are arrested by Baron Mordo and brought before the Illuminati. The Illuminati explain that their universe’s Strange used the Darkhold to defeat Thanos, inadvertently causing a multiversal "Incursion" that destroyed an entire reality; consequently, they executed him.
Using the Darkhold to "dreamwalk" into her Earth-838 counterpart's body, Wanda attacks Illuminati headquarters. She kills Black Bolt, Reed Richards, Captain Carter, Captain Marvel, and Charles Xavier.
Wanda captures America and teleports her back to Earth-616's Mount Wundagore, stranding Strange and Earth-838 Christine in a collapsed universe ruled by a corrupted Sinister Strange. Strange defeats Sinister Strange, retrieves his Darkhold, and performs forbidden dreamwalking into Earth-616. Because dreamwalking requires a body in the target universe, Strange possesses the rotting corpse of Defender Strange. Controlling Zombie Strange with spirits of the damned, he attacks Mount Wundagore.
Instead of attempting to overpower Wanda, Strange instructs America to trust her abilities. America opens a portal to Earth-838, bringing Wanda face-to-face with Earth-838’s Billy and Tommy. The children recoil in horror, viewing her as a monster. Horrified by her own actions, Wanda destroys Mount Wundagore and annihilates every copy of the Darkhold across all universes, bringing the temple down upon herself.
In the fallout, America remains at Kamar-Taj to train, while Strange repairs his broken watch. Walking down a New York street, Strange suddenly collapses in agony as a Third Eye opens on his forehead—a lasting consequence of his Darkhold usage. In a mid-credits scene, the sorceress Clea appears from a portal, inviting Strange to join her in repairing a multiversal incursion he caused.