Origins of the book used by Wanda in the post credit scene?

Question

In the post credit scene in the final episode of WandaVision Season 1, Wanda Maximoff (Scarlet Witch) is seen reading a book. What's the origin behind the book & what's it called?

Answer 1

The Darkhold (Book of the Damned)

In Doctor Strange in the Multiverse of Madness, we gain further insight into the nature and origin of the Darkhold. Previously described as the "book of the damned" by Agatha in WandaVision, Strange corroborates this characterization:

Strange: The Darkhold.
Wanda: You're familiar with the Darkhold?
Strange: I know it's the book of the damned...
And that it corrupts everything and everyone that it touches.

Wong reveals that Wanda's Darkhold was actually a copy, with the original spells carved into the walls of Mount Wundagore:

Wong: The Darkhold was a copy!
Wanda: A copy?
Wong: Legend speaks of a mountain with the wretched spells you seek carved into its walls.
It's there the Darkhold was transcribed.
Mount Wundagore.

The first demon, Chthon, is identified as the author of the Darkhold:

Wong: Eons ago, the first demon, Chthon, carved his dark magic into this tomb.
These same spells were later transcribed into the Darkhold.

The Darkhold allows "dreamwalking," as explained by Earth-838's Mordo:

Mordo: There is a spell contained within those pages...
Corrosive to the soul...
A desecration of reality itself.
Dreamwalking.
A dreamwalking sorcerer projects their own consciousness from their universe into another possessing the body of an alternate self.
They may puppeteer this unholy doppelganger and pursue their enemies from afar.
The possession is not a permanent link between realities.
But in the fleeting time they dreamwalk...
They can do irreparable damage to the universe they invade.

The Darkhold has an antithesis known as the Book of Vishanti:

Strange: What do you know of the Book of Vishanti?
Mordo: The Darkhold's antithesis?
It can give a sorcerer whatever power they need to vanquish their enemy.

It's revealed that versions of the Darkhold exist in other universes, with Earth-838 Illuminati Mordo and Sinister Strange guarding their respective copies.

At the conclusion of Multiverse of Madness, Strange states that Wanda destroyed the Darkhold in every universe. This is later confirmed by Rio Vidal, a witch, in the 2024 TV series Agatha All Along.

In Season 1, Episode 3 of Agatha All Along, it's revealed that Agatha traded her own child for her copy of the Darkhold, explaining how it came into her possession.

Answer 2

The book is the "Darkhold," or "the book of the damned," as mentioned earlier by Agatha when she is reading from it.

Agatha: Wanda, you've never been up against another witch before. Did you know there's an entire chapter devoted to you in the Darkhold? That's the book of the damned. "The Scarlet Witch is not born, she is forged. "She has no coven, no need for incantation."

_{WandaVision, Series 1 Episode 9, “The Series Finale”}

Apart from that, per the MCU we don’t really know much about it; it doesn’t show up in Doctor Strange if I remember correctly. It seemed Agatha was able to conjure it up though, so presumably Wanda got the book in the same way.

The book does show up in Agents of S.H.I.E.L.D. and Runaways, but as the canonicity of those shows to the MCU is a little debatable at this point, and I haven’t watched them, I’m not going to look into it here.