A Demon's Wrath is a brilliant side story to Sarra Cannon's Shadow Demons Saga, told by Jackson's, or Denaer's, POV (I'll call the protagonist Jackson going forward, as that's the name by which he is most well-known).
It provides Jackson's backstory, so far back that it ranges 101 years. It's the perfect companion to the first 6 books of the SDS. We finally learn how Jackson and Lea became promised to each other, how Aerden was stolen from the Shadow world and held hostage for decades inside a Brighton family prima, and how the Resistance was born. We find out how some of the newer characters in the saga came to be part of Jackson's life (Andros, Ourelia and Jericho). The best parts, in my opinion, are the scenes in which we see Jackson's first vision/drawing of Harper and how he is transported, through the Sapphire portal, to Peachville.
This novella falls dead center between the first and last 6 books of the Shadow Demons Saga. Though you can skip it altogether and simply read books 1-11 (she is currently writing book 12 as this is being written), I HIGHLY recommend making sure that you save time for A Demon's Wrath before reading Emerald Darkness. There are present-day scenes that happen immediately after Demons Forever that you DON'T want to miss!
Simply put, it's full of magic and mystery and answers. Answers that any fan of the SDS would have desperately been longing for.
This is an easy 5 stars for me.