Does anyone know if there will be new demons or other fully developed adversaries in this book? The average level of our group is about 18.5, so this seems like a very useful tool to begin an adventure in the abyss.
If you're referring to the Fiendish Codex, then it does have quite a few foes for you to use. There are at least 10 demon lords/princes fleshed out in the book (including some new ones like Dagon and Obox-ob) that are all CR 19+ and there are several new or updated demons, like the chasme, dybbuk, ekolid, moleydeus, nabassu, and sibriex. Most of these demons are CR 10+ or higher.
Even without the monsters, the book has a plethora of feats, spells, adventure ideas, and abyssal layers that you can use.
