First off- to my players- DON’T READ! Ok- now that has been said…
The characters are going to find out that a wizard has found a page of the Book of Vile Darkness (although it’s not a real page- a darker force has sent out copies of certain pages that corrupt the reader, driving them mad) and it shows how to tear open the Void and bring forth horrors.
I found the map at Maps of Mastery and just used Publisher to add the numbers and key it up.
Enjoy and feel free to leave feedback on how it worked for you!