A great book overall that adds spice to any underdark campaign, or rather any campign in all honesty, just more so in an underdark story.

The art is simply enchanting giving the Monsters of series a great identity due to it being done in a single artists unique style. The drow are a stylized yet not too foreign, the Svirfneblin have a look that surpasses their offical art in my honest opinion and the undervoid and its minions are a strange mixture of threatening and cute. Despite the more family friendly art i can easily imagine the beasts in a grim or gritty campaign. This product will give you minor to major baddies to face in glorious combat and roleplay!

The issues I have with this book though are few, first the infected creatures (Despite being part of the undervoid my favorite thing in the book.) all feel too similar in stats, and a tempalte block would have been more useful for converting monsters to the undervoid's service, and also the player options feel a bit tacked though they can be modified with some simple homebrewing. (also i can't quite search or highlight the text on my PDF copy, so i hope it gets updated eventally)

All in all monsters of the underworld will expand almost any 5e game in glorious ways, if you're running out of the abyss, then maybe spice things up with some lovely monsters from the deep, after all what's more unnerving than strange exotic creatures from beyond the base game?

A lovely product for fey focused DMs who enjoy the slightly darker aspects of those mythological monsters (well dark as in the more fun kind of dark, thinkPans labryinth or a Neil Gaiman Story). Along the twisting way is full of stat blocks for end game bossese to early level fodder making sure that you can keep using this product for all play ranges. The artifacts and locales will want players to keep exploring the Demesnes of the fey soverigns (and Red Jack) even as they have to fight the inhabitiants as well as the enviroments as well. Each chapter has a short little tale to set the mood and even a few excerpts from classical literature to hint at their inspiration.

This product however is mostly supplementary though, while it is full of locations and lore it lacks a main focus on the material plane, which is a plus since it leaves the DM ample room to focus on that aspect of a setting, and with that being said i suppose one could run a campaign entirely within the lands of along the twisting way they'll just need to be very mobile or start off in one of the more friendly Soverign's lands.

My only complaint is that this book references other Fey Soverigns like Titania and Oberon without them getting stat blocks or Demenses. but the book is full enough as it is with fey and i'm certain that in the future Scott will take care of them (I also hope maybe he could add in the lady of the lake as well)

So in short pick up this product if your players love the feywyld and if you wish the Monster manual as well as Volo's Guide to Monsters had more of your favorite creature type, because i sure as hell know i do.