Everything beyond that initial reference is the creation of later Live Team members. Unfortunately, I can't even guess who should be properly credited for that work.
That's not a question for me, I'm afraid. I invented the Book of Eibhil as a toss-away reference when I wrote "Our Great Work" for Frore. I wanted something that sounded old and ominous, and went with a shabbily disguised reference to the "Book of Eibon" used by H.P. Lovecraft .
Related to Sendings of Darkness is the Book of Eibhil. It was quite important to AC2, and later in the life of AC1 it became the focus of a story arc. What is the origin of this book? Who wrote it? Where does it get its power?