If you watch "Doctor Strange in the Multiverse of Madness" and have some questions about the Darkhold, the book Wanda Maximoff (Elizabeth Olsen) uses to enact her plans, we've got you covered. The book has a long legacy in the MCU, but audiences most recently saw it in "WandaVision." When Wanda's nosy neighbor Agnes was finally revealed to be the witch Agatha Harkness — and the true villainous mastermind behind Westview — Wanda found a strange old book sitting in her creepy basement, surrounded by tendrils of magic.
That book was the Darkhold, a text that had already appeared on multiple Marvel TV shows. In the comics, the Darkhold is an ancient text filled with spells, curses, and dark magic. In the MCU, it first served as a major plot device on "Agents of S.H.I.E.L.D.," where its onscreen history was revealed: a text made of dark matter, from the Dark Dimension, that contains infinite knowledge but also seems to have a dangerous and corrupting influence on those who read it. Worth noting as well: the Dark Dimension played a major role in the first "Doctor Strange" movie, so it makes sense that the Darkhold popped up in the sequel, too.
On "Agents of S.H.I.E.L.D.," the Darkhold was eventually used as part of the Framework storyline, in which the characters were trapped in an alternate, virtual dimension. Eventually, the android Aida attempted to use the knowledge of the Darkhold to create herself a human (or, rather, Inhuman) body and to remake the real world in the image of the twisted Framework virtual reality. Her attempts created a rift in Hell, and the Spirit of Vengeance, housed within Robbie Reyes (aka Ghost Rider), wanted to kill Aida and bring the Darkhold back to Hell. Robbie teamed up with Phil Coulson and his team to trick and destroy Aida, and Robbie ultimately opened up a portal back to Hell and took the book with him.
Somehow, the Darkhold was brought out of Hell by Morgan le Fay, one of the major villains on the MCU show "Runaways." In that series, Morgan attempted to use it to conquer Earth, but it eventually was used against her. After the Runaways and their allies Cloak and Dagger rescued one of their own from the Dark Dimension with the help of the Darkhold's magic, they then used it to cast a powerful banishing spell to get rid of Morgan.
Until "WandaVision," the Darkhold's whereabouts and ownership had been unknown. One of its key powers is the ability to essentially pave the way for its readers to create their hearts' desires, at any costs. The "instructions" in the book can teach its readers to build items that harness energy from other dimensions, making them almost impossible for anyone on Earth to understand.
"Doctor Strange in the Multiverse of Madness" added more to the mythos of the Darkhold. At the start of the movie, Wanda still has Agatha's copy of the book, and she's intent to use it to steal America Chavez's (Xochitl Gomez) powers and travel the multiverse until she gets her sons back. Eventually, the sorcerers, led by Wong (Benedict Wong), are able to destroy it. But Wong reveals that the book known as the Darkhold was actually a copy of spells written by Chthon, a mysterious entity. Those spells are carved into a castle at the top of Wundagore Mountain, and Wanda makes Wong take her there so she can read them right off the wall.
During "Multiverse of Madness," we learn of a technique in the book called Dreamwalking, which allows a user to possess their body in another universe. Wanda uses this to try to catch Dr. Strange (Benedict Cumberbatch) and Chavez, and Dr. Strange eventually uses the same power to fight Wanda from another universe.
At the end of the movie, Wanda destroys the castle and all copies of the Darkhold across all universes. But this being the MCU, there's always the chance it'll come back again.