Seems that J.K. Rowling isn't the only one that can create a book of beasts set in their setting. Ok, so Godbeasts isn't just a book of monsters, but is a new supplement for Age of Sigmar that has a bunch of new things inside it for the game. But still, it was the first thing I thought of when I saw the title.
Godbeasts brings you four new Times of War rules. These are new game effects, spells, and a new Triumph table. There's also new Battleplans (aka, scenarios) that add in the ability to play 3 and 4 player games (besides just a free-for-all or teaming up). They have also included twelve Warscroll Battalions. They reward you for taking certain lists that fit together thematically. Plus a painting guide and model gallery.
You can order your book now and they'll start shipping out on the 9th.