Hello, I recently finished reading Apocalypse, and was very intrigued by the mention of the Mortis Monolith. However, I did have one question: Which came first, the Mortis arc or Apocalypse? And I mean this in Earth time, of course. Did Denning write the book using information on Mortis from the show? Or did Lucasfilm make that arc to act as a prequel to the book, so to speak?

Apocalypse came out March last year, The Mortis arc about a year prior to that,

 

The FotJ series had been in development for a few years already, so clearly the character of Abeloth long predated the appearance of the Ones on the show. Though it's possible the authors simply thought it was a good place to tie the character into to better fit the EU. It's not the first time either, as even the Darth Plagueis book was able to give a cameo to Mother Talzin.