A nice supplement to Avernus, this will help in my game when the players go off the rails.

This monster manual supplement and its predecessor is, hands down, the most informative and helpful monster supplement I've seen and used, even edging out Volo's Guide to Monsters in my opinion. The author took the base models presented in the monster books by Wizards and expanded them, doubling (if not tripling) the content presented, giving you more tools to play with and including creatures that were sorely lacking in the official books. The diversity of the same creature that was in 4e is presented here, to this reviewer's relief, switching up things on my players when they already know the stats of everything in the books by Wizards. Of particular note is a comment the author makes in the supplement: including Aspects. Very few campaigns (in my experience, and from talking to other players) ever get to the level where they can conceivably fight a demon lord and win, thus including the avatar or mini-version of several powerful foes. The other statblocks are well thought out, diverse, and brings a fresh wind of change to the table, surprising the players and delighting them with something new. In short, this book is amazing in all ways. It deserves to see print, and the designers at Wizards would be remiss not to offer the author a job. It has everything I look for in a monster book, including short hooks in the monster descriptions to spin off your own adventure and challenging foes. This is a work of love, and it pays off. Buy this book and the first MM Expanded RIGHT NOW.

This is worth far more than it's listed, full of awesome and quirky magic items. There are plenty of things in here that should have been in the DMG, and fills many of the gaps, all delivered with Mordenkainen's dry voice. If you're a DM, buy this book now!