The Necromancy of Thay key is an item you've got to hunt down in Baldur's Gate 3 if you want to open the mysterious dark magic tome you find in the apothecary's cellar in the Blighted Village. To get to the book itself, you've got to jump through all sorts of hoops, like opening a secret cellar door, tricking a magical talking mirror, and disabling some traps around the book itself.
You've also got to open the barred door in front of the book—you can grab the Rusted Key to do this from a shelf in the laboratory's westernmost room. This is right by the lever that lowers the wall, letting you back into the previous area, so it's hard to miss. Even when you have the book, you still have to unlock it, and ultimately decide whether you want to destroy it or take its power. Here's where to find the Necromancy of Thay key, plus what choices you can make.
