One of the main plot elements of the finale to Wandavision is a mysterious book of old magic named The Darkhold. Unsurprisingly, this book comes from the comics. Specifically an issue of Marvel Spotlight featuring Werewolf By Night.
As it continued appearing in Marvel's magical corners, so too did readers learn more and more of its backstory. Billions of years ago, the elder god Chthon fled the Earth in fear of a much bigger, nastier elder god come to play. Before he did, he wrote down the worst spells he knew into parchments of indestructible flesh. Those collected pages, bound together, are known as the Darkhold.
Since its creation, The Darkhold has passed from magic user to magic user, and almost none of them have been nice. This thing has some pretty twisted magic scrawled on its pages, so it should also come as no surprise that things rarely end well for anyone who tries to use it for themselves.
Using this book is not for the faint of heart, as even in the comics, simply looking at these pages is enough to get your blacklisted from the larger magic community. As we saw poor Agatha find out in the flashback that kicked off episode 8. And the punchline is that the stuff Agatha pulled off with it isn't even the tip of the iceberg.
But we're most likely about to see what else it can do, as now Wanda has her hands on the Darkhold and, as many have theorized from the beginning, is planning to use it to scour the multiverse to find real life versions of her family. Which will lead into Multiverse Of Madness, which will of course lead into some other stuff down the road that's a bit too off topic.
Whatever comes next, The Darkhold is bound to play a huge part in the MCU's coming years, for better, and most definitely for worse.