https://marvelcinematicuniverse.fandom.com/wiki/Darkhold
Agatha called the book The Dark Hold. If you want to know more about this book, watch Season 4 of Marvels Agents of S.H.I.E.L.D and Season 3 of Marvels Runaways.
The Darkhold was created in the Dark Dimension and is made of dark matter. It is said to contain infinite knowledge, spells, formulas, information on how to create machines to harness energy from other dimensions, and has a full chapter dedicated to the "Scarlet Witch" myth. Over the course of history, many individuals searched for the Darkhold, including Red Skull, Daniel Whitehall, and Nick Fury. During the search for the book, S.H.I.E.L.D. created a file on it, including a draft based on a description of the book's design.
Part of its power is that it can be read in the readers native / first language.
The secondary ability of the book is to corrupt the reader's mind and create a dangerous obsession over the Darkhold. A person who has used the Darkhold for a long time may not want others to even touch it. Even a brief skim through the book can corrupt the reader's mind.
It has a special connection with beings who are related to the book itself, the dark matter, the Dark Dimension, and users of dark magic.
In Marvels Agents of S.H.I.E.L.D the book had been hidden in an abandoned house and was found by Joseph Bauer and his wife. They were scientists working on a project to create matter. They went searching for The Dark Hold and used it to make Quantum Batteries which harness quantum energy from another dimension. They were killed by a team member Eli Morrow after arguing about possession of the book. Eli Morrow used the book to create a Quantum Particle Generator, a machine that defied the laws of physics.
After Coulson and his team became involved, and took possession of the book, it was used by Aida and Holden Radcliffe to create an alternative virtual world called The Framework. Aida , a life model decoy /
Artificial
Intelligent
Digital
Assistant also used the book to built herself a real human body using dark matter.
The book was eventually returned to the dark dimension by Ghost Rider who was working with S.H.I.E.L.D.
In Marvels Runaways the book was acquired by Morgan le Fay, a witch who had escaped the dark dimension. She wanted to merge the dark dimension with earth. The Runaways instead used it to send Morgan le Fay back to the dark dimension.
Which then brings us, in the books timeline of owners to Agatha Harkness , WANDA VISION and now Wanda has the book.