Doctor Strange makes a catastrophic mistake in the Spider-Man films are set free to wreak havoc on the Marvel Cinematic Universe.
Wanda Maximoff (Elizabeth Olsen) reaches her final form as Scarlet Witch in powerful burst of Chaos Magic. Though she doesn't know exactly how she did it, Wanda ends up bringing back Vision and holding an entire town hostage in a world of sitcom nostalgia. The magic is so powerful that it attracts Agatha Harkness (Kathryn Hahn), an ancient witch who plans to steal Wanda's powers. In the end, Wanda defeats Harkness and rightfully takes her place as one of the most powerful characters in the MCU.
Strange's spell makes Wanda look even stronger. In WandaVision, Scarlet Witch holds the town of Westview hostage for weeks. Her magic creates whole new identities for an entire community, and she doesn't even know exactly how she does it. In comparison, Doctor Strange's attempt at wiping memories is relatively pathetic. Regardless of how successful his spell is in the film, ripping a hole in the Multiverse and releasing Spider-Man villains wasn't the intent. Yes, Parker interferes with Strange's spell, but that's not an excuse. Strange's miscues show just how powerful Wanda has become.
Even before Strange's spell, there were hints that Wanda was more powerful. In Avengers: Endgame, Wanda also battles Thanos, nearly killing him by herself. Thanos eventually orders an airstrike to help him out of the situation, but Wanda could've ripped him apart. Then, in WandaVision, Harkness notes that the Scarlet Witch has even more power than Doctor Strange's Sorcerer Supreme. Wanda successfully creating new lives for people while Strange struggles to wipe memories further proves her magical superiority.
Of course, other factors are also at play in creating the Multiverse besides Strange's Spider-Man: No Way Home misfire. In He Who Remains (Jonathan Majors), thus destroying the Sacred Timeline and opening the MCU to branched realities. Additionally, Wanda may have opened up the Multiverse by fully embracing her powers as Scarlet Witch. There's even a theory that the endings of Loki and WandaVision sync up to show they both impacted the future of Marvel. Still, if Wanda opened the Multiverse, it was because she became more powerful. It's not because she made a colossal faux pas like Strange.
